I just can't find it!!

Do 你 mean fitness (like buff 或者 really skinny) 或者 the basic fat to skinny scale?
Either way they are both under "basics" in create a sim. Once you've created your sim 你 can't modify those anymore. The only way to make your sim skinnier 或者 buffer is to have them work out 或者 swim. I guess to make them fat just make them over eat all the time.
你 can also get the a lifetime wish reward called the body sculptor (or something like that) that allows 你 to quickly change your sims body type.
Hope that helps!