I just can't find it!!

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