Since a proper listing of all modificiers related to a characters size isn't presented in the Core rulebook, I've compiled a table here for easy reference.
relevant rules are listed in the PHB (pag. 179 a.o. ) and the Bestiary (pag. 292-297).
| Size | Str | Dex | Con | Natural Armor | AC/Attack | CMB/CMD | Fly | Stealth |
|---|---|---|---|---|---|---|---|---|
| Fine | same | +2 | same | same | +4 | -4 | +2 | +4 |
| Diminutive | -2 | +2 | same | same | +2 | -2 | +2 | +4 |
| Tiny | -4 | +2 | same | same | +1 | -1 | +2 | +4 |
| Small | -4 | +2 | -2 | same | +1 | -1 | +2 | +4 |
| Medium | 0 | 0 | 0 | 0 | +0 | +0 | +0 | +0 |
| Large | +8 | -2 | +4 | +2 | -1 | +1 | -2 | -4 |
| Huge | +8 | -2 | +4 | +3 | -1 | +1 | -2 | -4 |
| Gargantuan | +8 | same | +4 | +4 | -2 | +2 | -2 | -4 |
| Colossal | +8 | same | +4 | +5 | -4 | +4 | -2 | -4 |
The changes in the table listed above are additive, thus a character switching size from medium to huge would get the following total changes:
+16 str, -4 Dex, +8 Con, +5 to Natural Armor, -2 to AC&Attack, +2 to CMB&CMD, -4 to Fly and -8 to stealth.