Ok, I will try to answer all of your questions.
Yes they do get that big, and bigger. I have a normal female bearded dragon who is 20inches long from snout to tail tip. And they get that big fast, she was 18inches by the end of her first year, give or take.
No, starting a beardie out in a ten gallon tank is never a good idea. Physically, yes is is big enough, but here is the problem. Bearded dragons like it HOT! They require a basking temperature of 100-110degrees. Since you have leo's you know all about thermoregulation, in a ten gallon tank there is no way to create that hot of a basking spot and still allow the lizard to thermoregulate. If one part of a ten gallon tank is 110degrees, the whole tank is that hot, and that will cook your dragon. Literally, it can kill them. You can however, start out in a 20gallon Long aquarium and that will last a few months. Maybe four months?
As far as adult size tanks, most pet stores and even ooks will say a 40gallon breeder is big enough for an adult bardie. I, and i think most people on this forum, disagree. I had my female in a 40breeder for awhile and she was cramped! She is now in a 90gallon and is perfectly comfortable, although i am thinking about moving to something bigger. If you are going to go with glass aquariums the smallest i would say would be a 75gallon.
No they do not bite, as a rule. Of course, ever beardie is different and you have to work with yours to keep it tame, but generally they are extremely docile and friendly.
As far as care, beardies are quite different from leos. They eat TONS more, for example, and also eatr fruits and veggies as part of their diet. They definately take more time, energy and money than leos, but i think it is all worth it as they are very rewarding animals. Keep doing lots of research, check www.beautifuldragons.com they have excellent care information and make sure you can commit to a beardie for its whole life before getting one. If you do choose a beardie though, i can assure you that you will not be disapointed!
Good luck!
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0.1 Bearded dragon
0.1 mexican kingsnake
1.0.2 Leopard Gecko's
0.0.1 Rose Hair Tarantula
1.0 BTS
0.0.1 Reverse Okeetee Corn
0.1 Bullmastiff
4.1 Cats