What's the correct form of the word "size" in this sentence?
Do you have these shirts in large size?
or
Do you have these shirts in large sizes?
This would depend on whether you're asking for a specific size (large), or if you're asking about the several sizes that are considered large (large, extra large, extra extra large, etc).
"Do you have these shirts in large size?"
This sentence indicates that you're asking for shirts in the size known as Large. In which case, it'd be better to remove size, as it is clearly understood regardless, or perhaps by saying "the large size."
"Do you have these shirts in large sizes?"
There is nothing wrong with this sentence grammatically, though perhaps saying "the large sizes" might clarify things a bit. Like I said before, this is implying that you're looking for multiple sizes. If you want the large size, then you'd be better off with the first example, modified as specified.
In American English, a more natural way to ask this question is:
Do you have this shirt in (a) large?
The "a" is optional.