In France, you will use different expressions depending on the cloth
For shirts :
You would say something like :
-Tu mets quelle taille de T-shirt ?
-Moi ? du L pourquoi ?
You may use the full words "Small", "Medium", etc... but I think it is less common.
You will use a number instead :
- 36 (commonly XS)
- 38 (commonly S)
- 40 (commonly M)
- 42 (commonly L)
You would say something like:
-Tu fais quelle taille en pantalon toi ?
-Je fais du 42.
Here, you also use numbers :
You will commonly say something like:
-Tu fais quelle pointure ? (or: Tu fais quelle taille en chaussure)
-J'ai des grands pieds, je fais du 46 !
As PapaPoule says, you may also use "Chausser" :
-Tu chausses du combien ?
As 200_success says, using "pointure" is more common :
-Tu fais quelle pointure ?
or a more polite form:
-Quelle est votre pointure ?
Introducing a size
To introduce a size, depending on the structure, you'll use "du", "en", "taille" or blank:
Je fais du XS (most common)
Je mets du S
Je m'habille en M
J'ai un Jean taille 38
Je mets des vêtements L (not very common...)
You may also hear "Ce haut taille [petit/grand]" when the announced size of the cloth (let's say "M") seems wrong according of what you use to see (seems like a "S"). You may often hear it from people who care about their size :)