L-size shirt or L-sized shirt

If you want to focus on the fact that it has been sized, then use L-sized shirt.

Naruhodo, as we say over here. So if I'm trying to emphasize the size of something (say, in a comparison), I should use "sized". (He ate a jumbo-sized hotdog in one bite.)
In this case I'm editing a table of specifications, with no comparison or particular need to emphasize. Will stick with the original writer's "L-size". Thank you for your very helpful response!

Dou-ita-shi-ma-*****

Adjective: It's an L-size. (What kind?)
Participle (adjective): It's an L-sized shirt. (What process?)

ka ku nin desu ga... (just checking...)
Does "process" mean the process of making the shirt?
The original text of my question comes from a chip inspection line spec, and the term "L-size" refers to the size of the tray used to hold the chips. So, to answer the question "What kind?", it seems "It is an L-size tray." works.
But for the process question, could we (should we) then s

If it's the size of the tray that's in focus, then I'd go with "an L-sized tray", and if it's the tray that's in focus, then I'd go with "an L-size tray".

L-sized is a past participle, and as such it extends to the noun a past process (e.g., torn sleeve: it was torn; Cf. tear sleeve: a kind of sleeve.)

Hope that helps.
Gambatte!

I think it is sinking in. Please let me know if I've got it right.
- when focusing on the size of the tray, "L-sized tray" (How big is the tray?)
- when focusing on the type of tray, "L-size tray" (Which tray is it?)
- when focusing on the process, "L-size tray Inspection Process"

Thanks again Casi for all your help. (Now I can finally get some sleep!)