The first would probably be understood, but the second would be more natural.

Thank you. So if I was talking to a shop assistant, could I say something like: I'm not sure which size I should go for, but I have most of your jackets in a small? (This one is probably not natural.)

or most of the jackets I bought here are ---- and then something?

Sorry, I'm a bit lost here.

The first one is not very natural, unless you are trying to say that you have a size small example of almost every style of jacket they sell. You could say something like:
Most of the jackets I've bought here have been size small/small size.
Most of the jackets I've bought here have been (in) a small.
I usually take/wear a small in your jackets,

Ann225I have most of my clothes in a small.

Hmm. I have most of my clothes in a small closet.

Thanks for the tips. I find it weird and incomplete as well but it was the only thing that came up on Google when I tried to look it up. That's why I asked. Thank you both again!:)

Just to be sure. I have this top in a small size (in size small) but I need it in extra small. I think that adding another 'size' to the end of the sentence would be a bit redundant. Would this on

I'd probably say I have this top in (size/a) small but I need it in extra small.

I see CJ's point, but, in the context of discussing clothing sizes, in a small seems perfectly natural to me.