My child usually wears a size 4T, so I purchased this set in a size 4 - 5. The shirt fit nicely, but the pants were just bizarre. They were loose and baggy throughout the leg and knees, and were a little bit long. But the waistband was so small, it looks like it should be on a pair of infant sized pants. She was screaming that she couldn't breathe, and I don't blame her - it was like having a heavy rubber band wrapped around her waist. If you look at the pictures I've included, the black leggings on the bottom are a size 4T. I put these leggings on top so you can see how tiny the waistband is compared to that of another size 4T legging.
If they had a normal sized waistband, they would have been fine. I don't know how to work around this issue, because if I go a size up they will be enormous in the rest of the leg. I feel like it's got to be some kind of glitch, but regardless, they simply do not work. Really cute set, though, so it's a shame.