I also believe that the universal 5 ft step is unrealistic, though I wasn't aware of all the implications that a variable single-step could bring to the game.
The possibility of melee-focused characters having an extraordinarily adverse time of dealing with very large creatures is both frightening and fascinating at the same time. Denying them their full attack when dealing with very large monsters is entirely realistic. But really, why should one expect anything different? And why should this be necessarily unacceptable?
Mind you, monsters capable of making 10' steps are not siginificantly less vulnerable to ranged fire than the ones who can make 5' steps. Only the melee fighters are 'cheated'.
There are monsters that the team's spellcasters cannot deal with very well (monsters with a high SR, and monsters immune to magic outright) and there are other monsters that the team's combat specialists cannot handle very well (incorporeal monsters, monsters with a high DR, monsters with a high AC, and so on). Being ill-equipped to handle a monster, outside of simply being too weak to face it and win, isn't anything new.
Truthfully, however, it's up to the DM to decide what kinds of monsters a party faces. If every monster in the game could do 10' or 15' foot steps and deny the party's mele fighters their full potential, the DM wouldn't have much a choice and very rightly you could say that the variable steps would be ruining gameplay. But there are plenty enough critters in the monster books of an ordinary size and a lot of power to back it up.
If you ask me, extraordinary size is almost as much of a liability as it is an asset. You get adjustments to Strength and Constitution, but any improvement to AC you may get from natural armor is partially offset by your size penalty and your reductions to Dexterity.
Since a lot of monsters seem to operate within a range of 20-40 speed, it seems reasonable to base step size on that statistic. I agree that size in of itself shouldn't really enter into it, although a monster's speed is presumably appropriate for its size and so would its step size be. I can't help but see this as no different from a melee fighter not being able to confirm critical threats on a very large monster if the monster's vitals out of reach (or happens to be undead or a construct). It would just be another part of the game, and a specific shortcoming that requires a well-round character (or a well-rounded party) to overcome.
Base speed: step size
0-5 feet: n/a (any movement requires a move action)
10-30 feet: 5 ft
35-50 feet: 10 ft
55-70 feet: 15 ft
75-90 feet: 20 ft
95-110 feet: 25 ft
etc.
If variable steps truly ruin the game for melee fighters, you could always adjust the other aspects of the system to match: Consider the boosts of speed that the barbarian and monk classes get. If you tie in a creature's step size to the above chart, a 20th-level monk with an 80 or 90 base speed can very easily retain their full attack against most monsters, regardless of the size differential. There's also the Dash and Divine Vigor feats. If you wanted to allow Dash to stack with itself or somehow fix it so Dwarves can still fight those big bad giants while retaining their full attack in spite of the giants' faster movement, so much the better. Or use the following feats:
Long Stride
Your limber limbs allows you to move around in combat more easily.
Prerequisite: Dex 13+, speed 10+
Benefit: Your step size increases by 5 feet.
Normal: You can only make a 5-foot step while taking a full-round action in a given round.
Special: This feat does not increase your base speed.
Greater Stride
Your agility in combat is superior to most others'.
Prerequisites: Dex 15+, Long Stride, Run.
Benefit: Your step size increases by 5 feet.
Special: The effect of this feat stacks with that of Long Stride. This feat does not increase your base speed.
You could do something similar for the other movement modes than walking & running.
The possibility of melee-focused characters having an extraordinarily adverse time of dealing with very large creatures is both frightening and fascinating at the same time. Denying them their full attack when dealing with very large monsters is entirely realistic. But really, why should one expect anything different? And why should this be necessarily unacceptable?
Mind you, monsters capable of making 10' steps are not siginificantly less vulnerable to ranged fire than the ones who can make 5' steps. Only the melee fighters are 'cheated'.
There are monsters that the team's spellcasters cannot deal with very well (monsters with a high SR, and monsters immune to magic outright) and there are other monsters that the team's combat specialists cannot handle very well (incorporeal monsters, monsters with a high DR, monsters with a high AC, and so on). Being ill-equipped to handle a monster, outside of simply being too weak to face it and win, isn't anything new.
Truthfully, however, it's up to the DM to decide what kinds of monsters a party faces. If every monster in the game could do 10' or 15' foot steps and deny the party's mele fighters their full potential, the DM wouldn't have much a choice and very rightly you could say that the variable steps would be ruining gameplay. But there are plenty enough critters in the monster books of an ordinary size and a lot of power to back it up.
If you ask me, extraordinary size is almost as much of a liability as it is an asset. You get adjustments to Strength and Constitution, but any improvement to AC you may get from natural armor is partially offset by your size penalty and your reductions to Dexterity.
Since a lot of monsters seem to operate within a range of 20-40 speed, it seems reasonable to base step size on that statistic. I agree that size in of itself shouldn't really enter into it, although a monster's speed is presumably appropriate for its size and so would its step size be. I can't help but see this as no different from a melee fighter not being able to confirm critical threats on a very large monster if the monster's vitals out of reach (or happens to be undead or a construct). It would just be another part of the game, and a specific shortcoming that requires a well-round character (or a well-rounded party) to overcome.
Base speed: step size
0-5 feet: n/a (any movement requires a move action)
10-30 feet: 5 ft
35-50 feet: 10 ft
55-70 feet: 15 ft
75-90 feet: 20 ft
95-110 feet: 25 ft
etc.
If variable steps truly ruin the game for melee fighters, you could always adjust the other aspects of the system to match: Consider the boosts of speed that the barbarian and monk classes get. If you tie in a creature's step size to the above chart, a 20th-level monk with an 80 or 90 base speed can very easily retain their full attack against most monsters, regardless of the size differential. There's also the Dash and Divine Vigor feats. If you wanted to allow Dash to stack with itself or somehow fix it so Dwarves can still fight those big bad giants while retaining their full attack in spite of the giants' faster movement, so much the better. Or use the following feats:
Long Stride
Your limber limbs allows you to move around in combat more easily.
Prerequisite: Dex 13+, speed 10+
Benefit: Your step size increases by 5 feet.
Normal: You can only make a 5-foot step while taking a full-round action in a given round.
Special: This feat does not increase your base speed.
Greater Stride
Your agility in combat is superior to most others'.
Prerequisites: Dex 15+, Long Stride, Run.
Benefit: Your step size increases by 5 feet.
Special: The effect of this feat stacks with that of Long Stride. This feat does not increase your base speed.
You could do something similar for the other movement modes than walking & running.