Every dog varies a bit, even in the same breed, but they are generally between a certain range. Female Beagles should weigh approximately 20-23lbs.
The best way to tell if your dog is under and overweight is by the "tuck" in the stomach and the ribs. If the tummy near her hind legs is really raised up, and there are ribs showing clearly, she is underweight. If her tummy is level or lower than her rib line, and you can't see or feel ribs, she is overweight. Ribs should be able to be felt, and maybe seen slightly (in some breeds). If it feels fatty, and it's hard to feel the ribs, then she is getting close to being overweight.
If you look overhead of your dog, she should look thinner towards her her rear end. The stomach shouldn't be level or even with the rib cage.
Hope that explains it good enough!
