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 would say "Do you have it in large?", especially if referring to clothes sold in Small/Medium/Large/Extra Large sizes - e.g. T-shirts and underwear.