is she a 13 inch or 15 inch? it's hard to say what her weight should be. if you can just feel her ribs and do not see any bones protruding then she is at a good weight. i don't think i have ever seen a beagle that wasn;t overweight, except for in dog shows. they are generally chow hounds and tend to be on the chunky side. we had a beagle / bassett hound mix at my work who weighed a disgusting 98 pounds. i would say based on not seeing her maybe between 15 and 20 lbs.
Beagles are sturdy little dogs, and generally they weigh between 20 and 25 lbs. If you can't feel your dog's ribs (or can barely feel them), and it has no tuck up to it's stomach, it's overweight. If the ribs protrude through the skin, and the points of the hip stick out, it's underweight. I've seen a lot more overweight beagles than I have ones that aren't. If you really are uncertain about whether your dog is overweight (regardless of breed) take it to the vet, they will be able to tell you.
The correct weight depends on which variety of beagle you have. A full sized, 15" beagle will weigh 25 - 30 pounds. A thirteen inch beagle might weigh around 18 -20 pounds. You want to be able to feel but not see the ribs. Your beagle should also have a "waist", giving a "coke bottle" appearance when viewed from above. The correct weight for your beagle will be influenced by it's sex, height, bone and muscling. You might try viewing the beagles on our web site, http://www.talbothillbeagles.com. Go to the "Our Beagles" page. Abby is a full-grown 13" beagle. DeeDee and Rosie are shown as adult 15" beagles. Tryst, Squirt and Red are shown as puppies. Perhaps comparing these dogs at correct weight will help you determine if your dog is overweight.