Hope your mom lets you keep them. I enjoy them as much as the snakes I think, sometimes.
Average litter size is 8-15.
As far as grades/sizes go:
Pinks: Anything without hair (can be small or large)
Peach Fuzzies: Just a hint of hair coming in
Fuzzies: Velvety sheen to full hair (eyes still closed)
Crawlers: Eyes may be closed or open, but they're actively crawling around the container on their own. May be eating small amount of solid food.
Hoppers: Eyes open, teeth developed, eating solid food but still nursing. And as the name suggests, they hop when you try to catch them.
Weanlings: 4-6 weeks of age, fully haired out and taking care of themselves by eating solid food and can reach the water bottle.
Pics of the grades/sizes can be seen with size references here:
Rodent Pro Mice
As far as euthanization goes, I typically freeze the pinks through fuzzies with the eyes still closed. It only takes them a matter of minutes before they expire. Leave them overnight before bagging them up.
The larger mice, I flick in the back of the head and let them "drain the urine" out in a paper towel, then arrange them out on a tray and freeze them before bagging them up. It keeps from getting weird shapes of mice when you thaw them out.
Hope that helps. If you have anymore questions, feel free to ask.