Either is correct. If someone asks "What size do you wear?", rather than repeat the word 'size' in your answer, you would just say "I wear a 16" or just "16". But if you stating what size you wear, without somebody asking you, you might say "I wear a size 16" or "Because I wear a size 16, I can't always find the clothes that I like", etc. You might also hear people say they are a size, for example, "I'm a size 16". Hope that helps!