I've never used/ordered leg bands so I can't help you with size either... I just use bags of colored zip ties (I get the neon colors), and numbered wing bands (these are poked thru the webbing of the wing). The zip ties (I either use just 1 or a combo of colors per bird to identify them by flock and/or sex) are great for an instant visual, but IMO the numbered wing bands are better for record keeping/flock data. I use a combo of both, but they both have their downfalls.
The downfalls to the zip ties are that they have to be cut off and changed out as the keets' legs grow, which requires catching the birds and frequent monitoring of how tight they are getting on the leg, and on adult birds they only last about a year before the sun breaks them down and they fall/break off.
The downfalls to the wing bands are that you need to wait until the keets are about a week or so old before you put them on (there's just not enough wing webbing to work with on keets before that age IME/IMO), and also that they require catching the birds to read the numbers on the band each time you need an exact ID on a bird.
The major plus to wing bands tho is that they are a one time application identification band, you put them on as keets and they will remain on the bird for life, you never need to change them due to growth and they never get ripped out (at least the 100+ wing bands that I've used haven't).