I hope the get rid of weapon "sizes". Instead, I would rather just have a "size damage" bonus when wielded by a Large or larger creature − once per round.
For example: if a Large creature wields a longsword it deals the normal onehanded damage 1d8 + 1d6 size bonus. A Huge creature deals 1d8 + 2d6. And Gargantuan 1d8 + 3d6.
The size bonus to damage is only once per round, representing lunging and inertia.
Oppositely, if a creature wield a wrong-sized weapon, the damage bonus still applies, but the attacks incur disadvantage.
By simplifying and standardizing size damage this way, it becomes easier to design balanced mechanics for Large player characters. And easier to estimate what to expect when confronting a larger creature.
five six seven eight nine ten eleven & twelve headed hydra
weretiger (all forms)
If you look at 3.5 MM294 you can use the table on the top right to add combat reflexes & a fort save bump with +1cr from a level of fighter or use your judgement with the "adding special abilities" section slightly below. It was very easy for the GM to add combat reflexes or even just swap one of the feat(s) nearly every monster had to give it combat reflexes. When a monster's entire purpose is to act as a football style linebacker that keeps the bbeg alive more than one round without needing to give the bbeg a zillion hp it's a blindingly obvious choice to give said monsters those kind of abilities too.
But I would go for 6 action points per turn(AP). for 6 seconds that the turn lasts.
then you can have 2 AP for a single attack. And 3 AP for dual wield attack.
Or 3 AP for "power attack"
Movement can be less, 15ft per AP.
Casting spells can be anywhere from 1 to 6 AP. Current bonus action spells could be 1 pt, while regular Action spells can default to 3 AP.
Then metamagic can add X amount of AP to casting spell to add effects or to remove components(V,S,M).
haste spell can increase AP pool by 1-3 points, depending on spell level, same as Slow,
having less AP is also good option for some Poisons, Diseases or exhaustion levels.