the size appears to be pretty much correct based on just quickly looking at the pics. i'm not sure how familiar you are with a Vikings jersey but the head patch is located just below the next line on the back of the jersey above the nameplate. you could go online and look for pics of Vikings jersey to see the patch and then try to get a better idea of its relative size.
I'm not expert on these things, but there isn't anything about it that really jumps out at me and screams fake. That being said I'd still definitely do as much homework as possible ahead of time to make sure you know what you're getting.