IIRC, all boxed Psygnosis original T-Shirts came in XL size. It should have a mark on at least one label saying that. 100% Cotton could be right too.
They have been copied many many times, with various labels. I never had one, but I've certainly seen a few.
The artwork is particularly awesome on that issue, and imitation is the sincerest form of flattery, but I could not be sure. Some counterfeiters go to a lot of trouble, the original artist or someone with access to it could have re-issued the work (happens sometimes, deals run out of time).
If "Screen Stars" got the rights to the game as a budget title, they could have reissued it, I guess. That would make it a Psygnosis T shirt, but not an orignal one. But a legal clone. Not a bootleg.