The company has so many designs that you are bound to find something's you like. I got some Dino shirts for my boys but wasn't sure about the sizes. I'd say they fit smaller than expected. My 6-7 yo boy would fit Small youth nicely is not a little snug, so best would probably be M. My 4-5 yo boy would usually be x small youth but that's snug too so small youth works better. And my 2-3 yo girl works well with 3T. My kids are average at and slightly taller than average child they're age.
So, what's really great is there customer service and policy. I contacted them to exchange a shirt and they said they'd send another shirt, appropriately sized without fee! It's great and will be back for more!