Can you fire Large arrows from a Medium bow?
Would you take a penalty on your attack roll? What would the damage be?
As I understand it damage is based on bow size, not arrow size (with an exception for what happens when magic is used).
Beyond that, as far as I am aware you must use the appropriate arrow size; an arrow designed to be fired from a medium bow for a medium creature dealing the normally stated damage. If dealing more damage was as simple as firing larger arrows everyone would be doing it.
Also enlarge person doesn't work since arrows reduce back to their normal size when fired.
What will work is buying arrows designed to be fired from a large size bow, casting enlarge person on yourself after dropping the arrows on the ground, pick up the arrows, and now fire your bow. Since the arrows aren't magical enlarged they will deal large size bow damage.
Can you fire Large arrows from a Medium bow?
Not normally, no.
Would you take a penalty on your attack roll?
Not usually possible. If you have a special ability to do this, see that ability.
What would the damage be?
When a ranged attack mixes size of launcher and projectile, the damage done is based on the smaller of the two.
If dealing more damage was as simple as firing larger arrows everyone would be doing it.
Well, dealing more damage with (say) a longsword is as simple as using a larger one, but not everyone does that because there are penalties involved on the attack roll and #hands. I was thinking that might apply to arrows too, but I guess not.
Thanks all!
Enlarge person is 1 min/level, so it doesn't have to be cast during the fight. Granted, the "drop Large arrows, cast enlarge person, pick up Large arrows" trick can work in some circumstances; it's just not going to be useful all the time. Especially with the -2 to ranged attacks (-2 Dex, -1 to all attack rolls for being Large).