“Can I try this on?”
Try something on means to put it on to see whether it fits or suits you. Pronouns go between the verb and particle: try this on or try it on. With a noun, try on this dress is also possible.
A1-40 / CEFR A1 / EPISODE 40
Trying on clothes and asking for a size
SceneAt a clothing shop.

WORDS & CONTEXT
Try something on means to put it on to see whether it fits or suits you. Pronouns go between the verb and particle: try this on or try it on. With a noun, try on this dress is also possible.
Here medium and small are nouns meaning items in those sizes. We do. is a short answer to Do you have …?, standing for We do have a small.
In a store, I'll take it. means you have decided to buy the item, rather than simply carry it away. I'll take this one. in A1-51 uses the same pattern.