It is how old they are which corresponds to how big they are. Ghann's has a size chart to give you an idea of how big the sizes are. This is how it's usually broken down:
1 week = 1/8"
2 week = 1/4"
3 week = 3/8"
4 week = 1/2"
5 week = 3/4"
6 week = 1" - adults: done growing and have wings so they're noisy!
No they'll grow steadily during that time. The sizes were based on their age according to week. So you can expect to grow each week or two up to the next size. But if you order them smaller/younger than you need them to be then you can get more at one time and let them grow some instead of constantly having to buy more of one size. If you have baby chams it may not be ideal because they'll get too big after a few weeks of growing. Does that make sense?
I have a 5 1/2 to 6 month old Veiled and she is eating adult crix. It just depends on how big your Cham is. I think the standard is that you don't want to feed them any thing that is larger than the space between the eyes of your Cham. I might be off on that though so I will leave it to the pros to correct me.