In France, you will use different expressions depending on the cloth

For shirts :

• XS
• S
• M
• L
• XL

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.

For pants

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.

For shoes

Here, you also use numbers :

• 36
• 37
• 38
• …

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 ?
-Du 38.

As 200_success says, using “pointure” is more common :

-Tu fais quelle pointure ?
-Du 38.

or a more polite form:

-Quelle est votre pointure ?
-38

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…)

Side note

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 🙂

In case you ever stop by Québec (Canada) afterwards, and wonder why it’s different.¹ There you will find, for example:

TP (très petit) [XS]
P (petit) [S]
M (moyen) [M]
G
(grand) [L]
TG (très grand) [XL]
TTG (très très grand)
[XXL]

For instance I’ve found M/M on a sweater with equal font size : usually you’ll have the French/English abbreviation on the label, as is the case here (moyen/medium i.e. M/M). My experience is mostly saying the English words in full here (du medium), especially for something like pizza (une extra large) and I don’t think I’ve ever heard anyone referring to sizes with the short form only in Québec (i.e. du M, du TP etc.). Combining both languages in full creates confusion (for me) as in un pantalon large (Eng. ample) vs. un pantalon (de taille) L (France, inferred from other answer). As for shoes, I’m only familiar with the verb chausser for wearing shoes of a certain size and pointure for referring to that size (the number of points).

_{¹ Because Québec. But there’s also the 1977 Charter of the French language (ch. C-11, RSQ) which states:}

_{51.Every inscription on a product, on its container or on its wrapping, or on a document or object supplied with it, including the
directions for use and the warranty certificates, must be drafted in
French. This rule applies also to menus and wine lists.}

_{The French inscription may be accompanied with a translation or
translations, but no inscription in another language may be given
greater prominence than that in French.}

_{So that one need not maintain in their mind some compatibility scale using a foreign language referential, or outright have to learn that language to buy clothing. There is also the flexibility for having accompanying translations/abbreviations in as many languages as whomever wishes alongside French.}