You need at least two hands to use a bow, regardless of its size. A greatbow sized for a Medium character is 6 feet or more in length when strung. A greatbow is too unwieldy to use while mounted. Like other bows, if you have a penalty for low Strength, apply it to damage rolls when using a greatbow. If you have bonus for high Strength, you can apply it to damage rolls when you use a composite greatbow but not a regular greatbow.