Hello, both answers would be understood by the salesman. However, I think that "I wear a size 16" is a more formal way to answer.

both are correct

Hello, Naomi I really appreciate your good explanation.

if you was in new york, you’ll be ight! 👌🏾...mostly everyone speak in short sentences sometimes due to “(Ebonics)”

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!