Since a proper listing of all modificiers related to a characters size isn't presented in the Core rulebook, I've complied a table here.
relevant rules are listed in the PHB (pag. ) and the Bestiary (pag. 292-297)
| Size | Str | Dex | Con | AC/Attack | CMB/CMD | Fly | Stealth |
|---|---|---|---|---|---|---|---|
| Fine | same | +2 | same | +4 | -4 | +2 | +4 |
| Diminutive | same | +2 | same | +2 | -2 | +2 | +4 |
| Tiny | -4 | +2 | same | +1 | -1 | +2 | +4 |
| Small | -4 | +2 | -2 | +1 | -1 | +2 | +4 |
| Medium | 10 | 10 | 10 | +0 | +0 | +0 | +0 |
| Large | +8 | -2 | +4 | -1 | +1 | -2 | -4 |
| Huge | +8 | -2 | +4 | -1 | +1 | -2 | -4 |
| Gargantuan | +8 | same | +4 | -2 | +2 | -2 | -4 |
| Colossal | +8 | same | +4 | -4 | +4 | -2 | -4 |