I just can't find it!!

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