If it doesn't have line weight markings on it, you might want to compare it to some other reel to get a sense of what's best suited to a reel that size. For example, if it's a medium sized reel like a 2500 Shimano or a 20 size Quantum, you're probably best off running 8-10lb test mono, which should give you 100-120 yard capacity and better line handling.
You could run 14-20 lb. test, or, conversely, 4-6 lb. test, but why bother if it's a medium sized reel?
Whatever you do, don't use braid, the reel was never designed foe that harsh line, and braid will likely cut a groove in the roller & bail, rendering he reel useless.