You would say "guapa pero gorda".
Gordo means fat in Spanish as an adjective.If you are talking about fat as a noun, than it would be grasa.
No gordo
Gordo
bonita
You can say "Eres bonita".
su grasa
You can say "Ella es bonita".
dedo gordo
cordo fue
estas gordo [thats a really mean thing to say] lol
You're not fat = Tu no eres gordo/a (male/female)
que es la grasa