Size

Miniscule - Fine - Diminutive - Tiny - Small - Medium - Large - Huge - Gigantic - Colossal - Titanic - Monumental - Awesome

Some Possible ideas: https://dnd-wiki.org/wiki/Expanded_Size_Categories_(3.5e_Variant_Rule)

Medium = 1 x 1 Large = 2 x 2 Huge = 3 x 3 Gigantic = 4 x 4 Colossal = 6 x 6 Titanic = 8 x 8 Monumental = 10 x 10 Awesome = 12 x 12

(check dimensions to be sure of the above)

From 3e: Creature Size / space / Natural Reach Fine / 1/2 ft. / 0 Diminutive / 1 ft. / 0 Tiny / 2-1/2 ft. / 0 Small / 5 ft. / 5 ft. Medium / 5 ft. / 5 ft. Large (tall) / 10 ft. / 10 ft. Large (long) / 10 ft. / 5 ft. Huge (tall) / 15 ft. / 15 ft. Huge (long) / 15 ft. / 10 ft. Gargantuan (tall) / 20 ft. / 20 ft. Gargantuan (long) / 20 ft. / 15 ft. Colossal (tall) / 30 ft. / 30 ft. Colossal (long) / 30 ft. / 20 ft.

Micro-X and Macro-X are used for sizes above this (going below Miniscule is Micro-Awesome, above Awesome is Macro-Miniscule)

Macro is for Mountain Sized, Mega is for Planet Sized, Giga is for Star Sized, Tera is for Solar Sized ( as per https://www.dnd-wiki.org/wiki/Revised_Size_(3.5e_Variant_Rule) )

(above gargantuan, increasing Toughness bonus per category? below tiny, penalty to-hit against? consider options)

(bonus to hit targets larger than gargantuan? penalty to hit targets smaller than tiny? make it "if a target is X steps bigger than you, +1 per step over that, penalty below? does this pose a problem for swatting mosquitos? what to do)

Damage

HP changes at sizes below Diminuitive and above Awesome; Fine HP = 1/2 a HP Each size category above Awesome, meanwhile, increases Hit-Dice Size by one step

Similarly, creatures below Tiny also do less damage (Diminuitive Damage equals 2 points per 1 point)

Move

Default Move for Tiny = Tiny Move 3 Diminutive = 3 Tiny = 6, for Small = 8, for Medium = 12, for Large/Huge = 12, Gigantic = 15, Colossal = 18, then +6 per size category.

move below diminuitive goes down like damage/hp