I have a little gut, but small legs. I've began running and exercising, but I don't want my legs to get any smaller, bigger if possible

Your legs won't actually get smaller, they will become lean and have great muscle defination. Do more then just running, you need to work out the whole body, add some resistance/weight training, and aerobics. Work out 30-60 minutes, 4-5 times a week, biking, swimming, dancing, all good forms of exercise. Also look into your diet, are you eating small portions of healthy foods, this is really as important as exercise, they go hand in hand in order to reduce body fat.

Bigger legs will not happen with running. You need to emphasize squats, lunges, and for cardio hit the inline skating not running. As for the belly, a lot of it has to do with body fat percentage. Diet is a huge factor in the tummy area. Hanging leg raises and complex crunch varieties will tone that area. To add bulk and drop bodyfat go with a higher protein, low carb diet. It takes work!