Light weight also means less lateral area in the water which generally means more maneuverability. The drawback to having a light boat is that it usually means a less stable boat. Length is a capacity and speed determinant of most designs. Longer boats generally have more buoyancy, translating to more capacity. Maximum hull speed of any displacement boat is determined by length.
Small sailboats are displacement boats up to the point where the boat starts planing.
So, until there is enough wind to get a boat planing, a longer boat will be faster. If you want a boat to sail solo you probably want something shorter than 15' so that it is not too big to maneuver by yourself and you can reach all the sail controls. Boats for two or more people are generally 14' or longer. Width determines stability more than anything, although more boats are using greater width toward the stern to help get the boat planing. All other things the same, a wider boat is going to be more stable than a narrow boat.