I think by donkey, you mean @$$, as in rear end. If you do, you would say, "¿Que tan grande es tu culo?". If you are actually talking about a donkey, you would say the same thing, replacing "culo" with "burro"
mula
Donkey and anus
He called her a "burro" which is spanish for a donkey.
horse= caballo sheep= ovejas donkey= burro
Viaja en el burro
He said she looks like a donkey!
Burro is the spanish word for donkey.
burrito (little donkey), pronounced 'booRREEtaw'
grande is big in Spanish. pronounciation (grondeh)
A burro is the Spanish name for donkey, a mule is a hybrid offspring of a donkey and horse.
Translation: El burro está en la cama.
grande