Now, your standard medium longbow does 1d8 and weights 3 pounds. Scaling it up 4 size categories means its now does 6d6 according to the damage dice progression chart and should weigh 3x2x2x2x2= 48 (originally used x8 from enlarge person, but YogoZuno pointed out on the weapons page). 12k pounds. Under the weapon rules, a large weapon costs 2x as much as a medium weapon, so said colossal longbow should cost 75x2x2x2x2=1200 (found on the weapons page and scrolling down to weapon qualities).
It is also worth mentioning that this weapon MIGHT also have a -8 on attacks roles due to the weapon being sized for a larger creature. Found on the weapons page by scrolling down to Weapon Size.
Inappropriately Sized Weapons: A creature can’t make optimum use of a weapon that isn’t properly sized for it. A cumulative –2 penalty applies on attack rolls for each size category of difference between the size of its intended wielder and the size of its actual wielder. If the creature isn’t proficient with the weapon, a –4 non-proficiency penalty also applies.
Now having shrink spell cast on this colossal longbow now makes it back into a medium sized weapon and weighs 48 / 4000 = 0.012 pounds. Since shrink spell makes no statements about what damage the weapon would do, we are forced to rely on spells such as enlarge person and reduce person, of which they do not agree on this topic.
Any enlarged item that leaves an enlarged creature’s possession (including a projectile or thrown weapon) instantly returns to its normal size. This means that thrown and projectile weapons deal their normal damage. Magical properties of enlarged items are not increased by this spell.
Any reduced item that leaves the reduced creature’s possession (including a projectile or thrown weapon) instantly returns to its normal size. This means that thrown weapons deal their normal damage (projectiles deal damage based on the size of the weapon that fired them).
So on one hand, reduce states its the weapon that matters, while enlarge says it does normal damage. So the best examples we have contradict each other for what happens. Since shrink item is more in tune with reduce person as both deal with shrinking, this would imply that a shrink item(colossal longbow) firing a medium arrow should deal 6d6 damage. Are there any rules, errata, FAQs that say otherwise? Crying DMs/GMs do not count.
There are two similar questions, but they ask different things. Shrink Item and the Oversized Starknife is asking about a thrown weapon. How viable is shrink item and massive projectiles? is asking about the projectile and not the launching weapon.