A.V.
Look for waists with the elastic adjuster hidden inside. That way you can get the length without the gap/falling down.
Generally my daughter has been a T per year in shirts, but it does vary with the brand, shrinkage, etc. Just yesterday she wore a shirt to bed that I was startled to realize was 4T (she's 2.5). She's just now getting into 2T pants so we still have 2 and 3 T in the closet.
I'm a big fan of leggings, too. Girls don't know how to play without flashing sometimes and leggings can get used a long time.