Toques: Generally fit true to size for the average child. The have a bit of stretch allowing them to keep a bit of structure but also fit across a bit of growth. They are also meant to have a bit of a slouch at the back. Generally I find that you want the width to be a bit less than the circumference of your child's head.
Sweaters: I make my sweaters with extra length in the arms and body (the part that usually does all of the growing) so that they generally fit for two sizes! The cuffs allow the arms to be a bit long but stay up. For example, a 1T will be on the big side (but should be wearable), but should also fit well at 2T, etc. If unsure, compare the measurements in the listing vs. an article of clothing with a similar fabric type that fits well.
Leggings: Generally fit true to size or a bit long. This gives some room for growth, or a bit of shrinkage if you're a fan of the dryer (I cant make myself put anything I love in the dryer for fear of any shrinkage!)
Washing:
(first wash) Cold wash low heat tumble dry. (after 1-2 washes) Warm wash, warm dry. No bleach.