I bought the 0-3 size for my 2-year-old.  I didn't use them much for the first half year because she would take them off, but she learned to keep them on around when she turned 3 and they help keep her from breaking her skin while scratching.  She is a small 3-year-old and the fingers were ok, but the thumb was already too short pre-washing.  Post-washing (in warm/cold, and drip dried) they are WAY too small.  Also, the fingers are on the slender side, but the palm is on the wide side, so the fit is not great.  Still the best we've found though.

Update: The size 4-6 worked well when my child was 3-4.5 but, post washing (air dried), the fingers are too short for my small 4.5 year old.  The fingers on the size 7-9 are a LOT longer, so a little ridiculous on my child, but she prefers it to fingers that are too short.  I think it will be at least a few years before she outgrows the 7-9.  I wish their size 4-6 had longer fingers.