What is a good size for my body type?

Ok I am a 14 year old girl with natural bleach blond hair and blue eyes. I currently am 5 foot 4 inches and weigh around 140 pounds. I am legit not fat, I have a German heritage so I have like a crazy amour of mucle. I am not lying at all I want answers. I wear a size 9 and a 1/2 shoe. I also wear a size 5 or 7 pants jeans or a small medium sweats. And last I wear a 36 B BRA. as you can see I am a bigger girl but not fat wise. I am just big boned and have a lot of mucle! But what I want to know is, in 6th grade I hit puberty and gained a significant amour of weight and was a little chubby. But I grew a few inches and lot the baby fat so my problem is fitting in pants. I have very big thighs with a lot of mucle but a little fat so I have issues with pants. My butt fits into a size 3 but my thighs don't. So I end up with a bigger size pants so I have to wear a belt or they fall down! Ok so my big question is what do u think a helthy weight and pants size to make my goal? I go to the gym and constantly am working doing lawns and also during school I do all the sports! So pleases be specific! A pant size and a good weight for my body type!! :) thanks