No question is silly - it's better to ask then be sorry. :)
They suggest to measure it right after the trim. However I run into the problem when you check the size right after the trim, but then have issues to put it on say 6 weeks later (I go on 8 weeks trim schedule and they grow pretty fast in summer). Because the boots are too small. :) So if I'd suggest to measure somewhere in mid between the trims and right after the trim. So you'll know how much they grow and won't order say size 1, but in 4 weeks your horse is size 2. You can still rasp off the edges without ruining the balance if your horse grew too much foot (if you know how to do that). Other boots (like Edge, Bare, Epic) are more forgiven if you go little bigger size, but from what I was told Gloves should be really like gloves so you have to have those ideal hoofs.