I am looking into buying a bike, but am unsure what size I should get. I am 5'9 female, slender. I just want to go riding on trails. Nothing off-road or anything.
Visit the local bike shop and they can help you out. It will depend on the bike you get. For something like rail trails and paved paths a hybrid bike will do better then a mountain bike. Take your inseam X .65 to get a rough estimate of the frame size you need.
