The sizing isn't incredibly inclusive for average or plus size people. I'm aware not everything caters to American sizes, but the "average" size globally increases yearly and the biggest size here barely fits a US 2X. I'm 5'6" and weigh about 265, and mei is naturally a thicker bodied character. I feel like these sizings should either run bigger or add more size inclusions.
HOWEVER, the costume itself is accurate, it's mostly sensory friendly and it's very cute. I find myself tempted to just wear this cosplay as pajamas. The shirt had long arms but it could definitely have more torso give, I had to flex it a bit to not hug me so much. but it wasn't uncomfortable. Just not super roomy like mei would've worn. The pants are super soft, and the outfit looks great for casual use. If you're looking for a premade costume/cosplay for pajamei, this is a SOLID option, even if I feel they could improve it.
