You don't change your stat's by appearances. The only thing that gives effects is the creature size. This matters for knockdown and ac for example.
Lizard is size 3, human too. Means they have the same bonuses and penalty's. Dragon for example is 5, gaints are 4, dwarfs are 3, halflings are 2, chickens are 1.
Servers like gs, arivs, lotr do check for this.
-Mel