Let's start with a mistake you hear all the time: be careful to not say "une assiette" (a plate) instead of "un siège" (a seat). Students get confused because the verb for "to seat" is "s'asseoir", so they think "une assiette" is related. Hence the mistake.

A Plate = Une Assiette

We have different kind of plates used for different courses:

Les Assiettes Plates (Flat):

une petite assiette (une assiette à fromage, une assiette à dessert par exemple) - smaller plate used for cheese or dessert for example.

une grande assiette (une assiette à entremet) - a bigger plate, used for the main course.

une assiette à pain - a very small plate for the bread

Note that a very small plate to put under a cup is called "une soucoupe".

Les Assiettes Creuses (Deeper Plate)

une assiette à soupe: soup plate

Les Plats (Serving Dishes)

There are too many to list: des plats creux (deeper), des plats plats (yes, "flat" serving dish), and we often sort them by their shape or use : un plat rond, oval, carré (round, oval, square...), un plat à poisson (for the fish), un plat à tarte (pie)... un plat pour le four (for the oven).

Ne Pas Être Dans Son Assiette

This weird idiom means to not feel/look well, to feel/look depressed.

Et bien, Camille, ça va ? Tu es sure ? Tu n'as pas l'air dans ton assiette.

Well, Camille, are you OK? Are you sure? You don't look well.

And it has nothing to do with a plate! Actually, it does come from "s'asseoir", and has to do with the position one is sitting: "L'assiette". It's an old French word, that nowadays is only used for horseback riding. We say: "un bon cavalier a une bonne assiette". (a good rider has a good sitting position). Otherwise, the French word "une assiette" is used for a plate, that's all.

Note that for the idiom "ne pas être dans son assiette" will always be used in the negative, and the possessive adjective will change to agree with the person you are talking about.

Regarde Pierre : il n'a pas l'air dans son assiette.

Look at Pierre: he doesn't look well.