Thanks again Hyper. It appears that the 3.0 Arms & Equipment Guide has a couple of tables on page 4 that show size increment damage for weapons. Looks pretty consistent with the 3.5 stuff when figuring out the the conversions for 3.0 to 3.5 weapons.
The 3.5 weapon charts list Dmg (S) and Dmg (M). It looks like you can use the Arms & Equipment Guide Damage Step Increase and Decrease tables to figure out 'damage step' increases and decreases - They seem to match what is in the 3.5 books even though they are 3.0.
The only one that I'm not sure about is the Blowgun, which has a 3.0 Damage of 1 point.....If you want to make a Small Blowgun for 3.5, then it's damage would be decreased one step, but the Arms & Equipment Guide tables logically do not show how you would decrease 1 point of damage. A normal blowgun does 1 point of damage. Would a small blowgun still do 1 point of damage, or would it do negligible damage (0 points) ?