For colloquial English, "Do you have a large?" is a better translation than "Do you have a large size?" - the word "size" is usually only used in eithet comparative (do you have a bigger size?) or superlative (is this the biggest size you have?).
I think it should be translated in English as: Do you have IT in a size large? or Do you have IT in a large size. Or do you have any big sizes? "Do you have a big size" does not sound natural to me (from USA).