
Lost In Limbo |

The rune giant would become Colossal+, and continue to advance as we see on tables 2-2 and 2-3 of Bestiary 1.
From a quick look, going from Colossal to Colossal+ would give a creature the following things, all size bonuses or penalties;
From 2-2, additive bonuses
- +8 Str
- +4 Con
- +6 Natural Armor
From 2-3, Total Bonuses and Penalties
- -16 AC/Attack
- +16 CMB/CMD
- -10 Fly Skill
- -20 Stealth Skill
Or at least that makes sense to me.

Samasboy1 |

Rune Giant
Rune Giant CR 17
LE Gargantuan humanoid (giant)
Init +0; Senses low-light vision; Perception +29
DEFENSE
AC 30, touch 6, flat-footed 30 (+9 armor, +15 natural, –4 size)
hp 270 (20d8+180)
Fort +15, Ref +6, Will +20
Immune cold, electricity, fire
OFFENSE
Speed 35 ft. (50 ft. without armor); air walk
Melee mwk longsword +27/+22/+17 (4d6+22/17–20) or 2 slams +26 (2d6+15)
Ranged mwk spear +12/+7/+2 (4d6+15/×3)
Space 20 ft.; Reach 20 ft.
Special Attacks command giants, runes, spark shower
Spell-Like Abilities (CL 20th; concentration +24)
Constant—air walk
At will—charm person (DC 15), suggestion (DC 17)
3/day—mass charm monster (DC 22), dominate person (DC 19)
1/day—demand (DC 22), true seeing
STATISTICS
Str 41, Dex 11, Con 28, Int 14, Wis 23, Cha 18
Base Atk +15; CMB +34; CMD 44
Feats Awesome Blow, Critical Focus, Improved Bull Rush, Improved Critical (longsword), Improved Vital Strike, Iron Will, Power Attack, Quick Draw, Staggering Critical, Vital Strike
Skills Acrobatics +15 (+23 jump), Craft (any one) +25, Knowledge (history) +12, Knowledge (nobility) +12, Perception +29
Languages Common, Giant, Terran
The Giant simple template tells you what benefits it provides
Quick Rules: +2 to all rolls based on Str or Con, +2 hp/HD, –1 penalty on all rolls based on Dex.
Rebuild Rules: Size increase by one category; AC increase natural armor by +3; Attacks increase dice rolled by 1 step; Ability Scores +4 size bonus to Str and Con, –2 Dex.
Gargantuan Colossal +8 Str Same Dex +4 Con +5 NA
Colossal –8 AC/Attk +8 CMB/CMD –8 Fly –16 Stealth
Using the rebuild rules, that gives us.
Rune Giant CR 18
LE Colossal humanoid (giant)
Init +0; Senses low-light vision; Perception +29
DEFENSE
AC 31, touch 2, flat-footed 31 (+9 armor, +20 natural, –8 size)
hp 310 (20d8+220)
Fort +17, Ref +6, Will +20
Immune cold, electricity, fire
OFFENSE
Speed 35 ft. (50 ft. without armor); air walk
Melee mwk longsword +27/+22/+17 (6d6+27/17–20) or 2 slams +26 (6d6+19)
Ranged mwk spear +12/+7/+2 (6d6+19/×3)
Space 30 ft.; Reach 30 ft.
Special Attacks command giants, runes, spark shower
Spell-Like Abilities (CL 20th; concentration +26)
Constant—air walk
At will—charm person (DC 15), suggestion (DC 17)
3/day—mass charm monster (DC 22), dominate person (DC 19)
1/day—demand (DC 22), true seeing
STATISTICS
Str 49, Dex 11, Con 32, Int 14, Wis 23, Cha 18
Base Atk +15; CMB +42; CMD 52
Feats Awesome Blow, Critical Focus, Improved Bull Rush, Improved Critical (longsword), Improved Vital Strike, Iron Will, Power Attack, Quick Draw, Staggering Critical, Vital Strike
Skills Acrobatics +15 (+23 jump), Craft (any one) +25, Knowledge (history) +12, Knowledge (nobility) +12, Perception +29
Languages Common, Giant, Terran
Spark Shower DC increases by 2
Enlarge Person also provides its benefits
There isn't a size category above Colossal. I know several 3.5 monsters had size listed as Colossal+, as suggested by Lost in Limbo. So we have 3 possibilities.
A) No effect, you are already maximum size
B) You get bigger, with the benefits of the spell, but stay Colossal as maximum size so no larger weapon damage, CMB/CMD, etc.
C) You become Colossal+, gaining the benefits of the spell and weapon damage increases by one step
I am going with B, you get bigger, stay Colossal, but otherwise get the effect of the spell.
Enlarged Rune Giant CR 18
LE Colossal humanoid (giant)
Init -1; Senses low-light vision; Perception +29
DEFENSE
AC 30, touch 1, flat-footed 30 (+9 armor, +20 natural, -1 Dex, –8 size)
hp 310 (20d8+220)
Fort +17, Ref +5, Will +20
Immune cold, electricity, fire
OFFENSE
Speed 35 ft. (50 ft. without armor); air walk
Melee mwk longsword +28/+23/+18 (6d6+30/17–20) or 2 slams +27 (6d6+20)
Ranged mwk spear +11/+6/+1 (6d6+20/×3)
Space 30 ft.; Reach 30 ft.
Special Attacks command giants, runes, spark shower
Spell-Like Abilities (CL 20th; concentration +26)
Constant—air walk
At will—charm person (DC 15), suggestion (DC 17)
3/day—mass charm monster (DC 22), dominate person (DC 19)
1/day—demand (DC 22), true seeing
STATISTICS
Str 51, Dex 9, Con 32, Int 14, Wis 23, Cha 18
Base Atk +15; CMB +43; CMD 53
Feats Awesome Blow, Critical Focus, Improved Bull Rush, Improved Critical (longsword), Improved Vital Strike, Iron Will, Power Attack, Quick Draw, Staggering Critical, Vital Strike
Skills Acrobatics +15 (+23 jump), Craft (any one) +25, Knowledge (history) +12, Knowledge (nobility) +12, Perception +29
Languages Common, Giant, Terran
But you definitely don't get the benefits of a true size increase, because that's not what Enlarge Person gives you.