Sure. But this isn't pnp.
In pnp, there is a "small" version and a "medium" version of every weapon. So there are small heavy crossbows that do less damage than medium heavy crossbows, of course.
But... there are also different sized longswords, and shortswords, and warhammers, and longbows, and daggers, and scythes, and battleaxes,... blah blah blah.
Do we really want to go down this long path of creating a "small" version of every weapon out there, just so some random halfling character can't do 1d10 damage with a crossbow?
This is really a non-issue in my opinion. Do we want to create a small version of every weapon or do we want to punish the tiny percentage of our player base who'd create a small character who uses a heavy crossbow?
In NWN, they just assign a single size to each weapon...
http://nwn2.wikia.com/wiki/Weapon_sizeHeavy crossbows are medium weapons. Small characters can use medium weapons if they use it two-handed.