OK, if we're using a size cut-off (which I'm setting at Large whether Shade likes it or not

) we'll need to modify the size & HD entries and both tables in the template.
Like this:
Size and Type
The base creature's type remains the same.
If the base creature is Fine or Diminutive in size the mystaran gargantua is Large-sized, if the base creature is Tiny or larger the creature's size increases by three size categories.
If the base creature is Huge-sized, the mystaran gargantua will be Colossal-sized with a Space of 40 ft. and a reach of 40 ft. if tall, 30 ft. if long. If the base creature is Gargantuan or larger in size, the mystaran gargantua will be Colossal-sized with a Space of 50 ft. and a reach of 50 ft. if tall, 40 ft. if long.
Hit Dice
If the base creature has 2 or more HD, multiply the base creature's Hit Dice by 8. For mystaran gargantua with fewer than 2 HD, the gargantua will have 12 HD if the base creature has 1 HD, 9 HD if the base creature has 1/2 Hit Dice and 6 HD if the base creature has 1/4 Hit Dice.
Damage
A mystaran gargantua retains all the natural weapons, manufactured weapons and weapon proficiencies of the base creature. The damage values of these attacks increase according to the following table (this includes improvement for increased size).
Mystaran Gargantua Damage
Old Damage . . New Damage by Base Creature's Size
(Each) . . . . Fine . . Diminutive . . Tiny or larger
. 0 . . . .. . 1d8*. . . 1d6* . . . .. . . 1d4*
. 1 . . . .. . 1d10 . . . 1d8 . . . . . . . 1d6
.1d2 . . . . . 1d12. . . 1d10 . . . .. . . 1d8
.1d3 . . . . . 3d6. . . . 2d8 . . . . . . . 2d6
.1d4 . . . . . 3d8. . . . 3d6 . . . . . . . 3d6
.1d6 . . . . . 4d8. . . . 4d6 . . . . . . . 4d6
.1d8 . . . . . 4d8. . . . 4d8 . . . . . . . 4d8
.1d10 . . .. . 4d10. . . 4d10 . . . . . . 4d10
.1d12 . . .. . 4d12. . . 4d12 . . . . . . 4d12
.2d6 . . . . . 8d6. . . . 8d6 . . . . . . . 8d6
.2d8 . . . . . 8d8. . . . 8d8 . . . . . . . 8d8
*If the base creature has an attack that deals no damage (such as a roper's strands), the attack does damage according to the "0" old damage row plus half the mystaran gargantua's damage Str bonus.
If the base creature has no attacks listed (such as a bat), it gains a single attack with the damage given in the "0" old damage row plus 1½ the mystaran gargantua's damage Str bonus. Use whichever type of natural attack seems most appropriate; it is usually a bite, slam or kick.
I'll have to leave the
Mystaran Gargantua Modifiers table for later.