You can tell the sex by the tail. Girls have petite little tails, and boys have much bigger ones. Also, the cloaca on a girl's tail is star shaped, but a boy's is more just a slit. There are some good pictures on www.russiantortoise.org that might help. As adults, they can be up to 10 inches.
The feeding schedule I've been following with mine is to give her as much as she can eat in 20 minutes every day.