Is Deidara a boy or a girl?
Deidara is a man. He looks like a man, uses pronouns and a way of speaking that is pretty much exclusive to men, and has male voice actors in English and Japanese. While it's not uncommon for women to voice male characters who are young/immature, it is rare for a man to voice a female character. 

There is no reason to think Deidara is a girl, and overwhelming evidence to suggest he is a man.