"The bird eats a big walnut."
Translation:L'oiseau mange une grosse noix.
This may help you : http://french.about.com/od/grammar/a/adjectives_4.htm
I'm no expert (in fact I'm actually pretty stupid at French) but I've read multiple times on other forums there's a rule of thumb known as B.A.G.S (Beauty, Age, Goodness (or bad), Size). If the adjective falls into any of those categories it should go in front of the noun. Anyone more experienced feel free to correct me if I am wrong :)