Found this from old offical 3.5 D20. Good reference material.
Table: Larger and Smaller Weapon Damage
Example Weapon Fine Diminutive Tiny Small Medium Large Huge Gargantuan Colossal
Shuriken — — — 1 1d2 1d3 1d4 1d6 1d8
Gauntlet — — 1 1d2 1d3 1d4 1d6 1d8 2d6
Dagger — 1 1d2 1d3 1d4 1d6 1d8 2d6 3d6
Shortspear 1 1d2 1d3 1d4 1d6 1d8 2d6 3d6 4d6
Falchion 1d2 1d3 1d4 1d6 2d4 2d6 3d6 4d6 6d6
Longsword 1d2 1d3 1d4 1d6 1d8 2d6 3d6 4d6 6d6
Bastard Sword 1d3 1d4 1d6 1d8 1d10 2d8 3d8 4d8 6d8
Greataxe 1d4 1d6 1d8 1d10 1d12 3d6 4d6 6d6 8d6
Greatsword 1d4 1d6 1d8 1d10 2d6 3d6 4d6 6d6 8d6
This is what we have been using from Paizo for Pathfinder:
Damage dice increase as follows: 1d2, 1d3, 1d4, 1d6, 1d8, 2d6, 3d6, 4d6, 6d6, 8d6, 12d6.
A weapon that deals 1d10 damage increases as follows: 1d10, 2d8, 3d8, 4d8, 6d8, 8d8, 12d8.
The Paizo follow the Longsword/Bastard Sword progression exactly. Which means this table from 3.5 should be valid and can answer some issues/question regarding size alterations dealing with polymorph buffs, pets, and Enlarge.