Hi! I was just at UNIQLO yesterday haha! I didn't buy or try anything on from their Nintendo collection, but I did look at them all. I would say that they are pretty normal sized. Yes, in Japan their clothing runs small because of our different bone structure/etc, but I think UNIQLO did a good job making sure the clothing will work well with their western markets.
I would say get the size you would normally in another store. As I said, I didn't try on any of the Nintendo collection shirts, but I tried and bought shirts from UNIQLO's other collaborations. My normal clothing size is small, but for the things I bought, they were mostly a little larger (longer?) than I expected, and for 1 of the shirts I bought, I had to get an XS, and though it fits me well in terms of width, it goes almost to mid thigh, and I (because I wore it today!! hehe) tucked it into my shorts. Overall, I don't think their clothing will run small, and I think you'll be just fine with your normal sizing.
Good luck!!!! And if you have any more questions about what they looked like (compared to your normal sizing or anything), just let me know

as a seasoned UNIQLO shopper, I'll do my best to help you out hehe