Why thank you Sheepies! Yes, the pattern is adorable. I plan to buy a matching shirt and also a pair of shortalls soon from Baby-Pants. As to your question, if I get a t-shirt, I usually get at least a 2X - 3X, often as large as a 4X - I like my shirts to be large and roomy.
If I wear a dress shirt, I need at least a size 20 neck, so that would be around an XXL size. I often cannot button my collar on a dress shirt, due to having a large, thick neck.
Baby Pants, I have found, typically sizes their onesies one size too large. I've tried the 4X, 3X, and 2X sizes already, and I've found that the 2X size is just about perfect. Their onesies tend to shrink by one size, so please bear that in mind. For reference, I wear a 46" pants, but my largest waist size is 56" around my stomach. I've found that the 2X onesies from Baby Pants fit me just snugly enough to be comfortable, and yet also provide some support, and keep my diaper from slipping down a bit, which is one of the reasons I buy them. My jungle friends onesie is really the first time I've bought for fun, and for purpose of embracing my Little side, as well as practical and functional reasons.
If you didn't know, the general rule of thumb regarding diaper fit is to go with the largest measurement, whether that is your hip/pant size, or your measurement around the largest part of your stomach. I think you can use a similar rule of thumb to size the onesie you want to get as well. I hope I helped you some. Thanks for your post!