See http://skmi.science.upjs.sk/~novotnyr/adnd/srd/spellsPtoR.html#polymorph Polymorph Spell in SRD3.5.

Polymorph:

• you can change to creature with size category at most one larger than yours size category
• you can change to creature with Hit Dice no greater than your Hit Dice
• your Hit Point does not change (even if the new Con is larger than your normal), however upon change to creature you gain Hit Points as if you slept for a night
• Str, Dex and Con may affect Save and Attack Bonuses