In the CRB, Equipment gives a table for weapons from small to large: I require from medium and beyond, including colossal. (I really hope there isn't one somewhere obvious in the book: I did my best to find it before posting this.)
There isn't one. Your best bet is to use the table that is from DnD3.5 HERE. This table did not get transferred into pathfinder but the scaling of what did transfer is the same. So this table should be correct or close enough to correct for any but the most RAW only players.
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That table seems to have a lot of contradictions.
Greataxes (1d12) and greatswords (2d6) both reduce to 1d10 when going from medium to small. So what does a Medium 1d10 weapon do in large form? It does 2d8. The 2d4 Falchion enlarges to 2d6, and so does the 1d8 longsword. But a 2d6 greatsword doesn't reduce to 2d4 or 1d8, but 1d10.
There's no internal consistency! You can't look at a specific set of damage dice and figure out what they'd do if enlarged or reduced-- you can only go by "what that weapon does when it's Medium".
I guess for most situation that's good enough, but it falls apart if you look at it too closely.
And yet...so same inconsistencies are in Pathfinder rules also.
Take a look at the greatsword for medium (2d6) vs small (1d10). Now look at the tiny and large weapon table HERE and you notice that a medium 1d10 weapon goes to 2d8 when large.
The same exact inconsistency. You just noticed it because its in a nice table on the d20srd but is split up and harder to notice in pathfinder.
