In 3.0 as far as I know, the max was +4. In 3.5, however, it scales upward infinitely, assuming you can find a craftsman to make it. Costs 100gp per plus in addition to the bow cost, so a +10 mighty bow would be 1000 gp beyond normal price.
I'm pretty sure there is no limit. Though I don't have any resouces with me at the moment. I seem to remember some centaurs with mighty composite bows +8 though . . .
Im playing a halfling about 2 and a half feet tall im using a sling as a ranged weapon and skipping stones. But theres not enough weapon verity for slings and skipping stones. Was wondering if you all could help out a shorty like me with some ranged weapons .
Part of the logic (IMO) for the no limit to Str rating for composit bows is that all composit bows ahve a STR rating in 3.5. It is not a special non-magical property that is added to the bow like it was in 3.0. If a character doesn't ahve sufficent Str to take advantage of the rating for the bow - in 3.5 he suffers penalties while in 3.0 he could only add his STr modifier and not the full bonus rating of the weapon.
The increase (i.e., scaling) of costs by Str rating would account for any special type of materials required to make the weapon without specifiying them in detail, at least IMO.