Have you seen images from Toma-san website? Theese are instructions - text is strange font, most people can't read it, but pictures are very clear
For 3yo http://www4.ocn.ne.jp/~tomasan/753onna3.html
For 7yo http://www4.ocn.ne.jp/~tomasan/753onna7.html
Fancy musubi for 7yo http://www4.ocn.ne.jp/~tomasan/obi7.html
Tucks diagram http://www4.ocn.ne.jp/~tomasan/age.html
As for shoe sizing, I haven't read anything specific, I think the easiest way is to look at neatly dressed girls
I browsed in "shichigosan" photos by google, and when it comes to zori, they are not always visible, but I think, that sizing is not that important. Some of girls have them too big, some to small (and they are all very neatly dressed, so I don't think it's a mistake). I guess there are a few fixed sizes (just like for adults - it's difficult to find something different than 23-24cm), so girls who grow faster have them shorter than their feet, and those more petite have them longer.