These run so short that they are more like knit pants than leggings for tall/thin builds. Size 8 has a comparable inseam to 5T pants, size 6 is comparable to 4T.

My son is ~45" tall and slightly under 40lbs, size 8 is the perfect length. They are loose but don't fall down unlike most 5T pants. They wash/dry well on cold and are very soft.