A four letter word and the opposite of fat would be thin.
A four-letter word for cooking fat is "lard".
skinny
fat
Successfully is a 12 letter word with three set of double letters. Stubbornness is another 12 letter word.
lard
Flab.
Brown recluse is a 12-letter word for a type of spider.
It is thin.
Suet
Lighthearted is a 12 letter word. Lithographed is another 12 letter word that starts with L and ends with D.
manufactured