[RPG] What size are the various races

A bullet shot from a small-sized Staff Sling deals 1d6

Just as the table indicates, using the weapon in this fashion deals 1d6 points of damage.

The Medium is for other users and corner cases

For example, if you cast Enlarge Person on the halfling, that damage value becomes relevant. A human who masters the Staff Sling would use a medium one as well.

Yes.

The soulknife ability clearly states that if a soulknife has Powerful build the knife it manifests is larger to match the ability. So you would deal increased damage due to having a larger knife than say.. a regular human.

By design this also means if you can find a way to get Enlarge Person to increase your size, then you would also benefit from the soulknife damage increase granted by powerful build as well.

Form Soul blade states:

A soulknife must choose the form of her mind blade at 1st level. She can either form it into a light weapon, a one-handed weapon, or a two-handed weapon. Once chosen, her mind blade stays in this form every time the soulknife forms her mind blade. The light weapon deals 1d6 points of damage, the one-handed weapon deals 1d8 points of damage, and the two-handed weapon deals 2d6 points of damage. All damages are based on a Medium-sized creature wielding Medium-sized weapons.

This constrains the Soul blade to a certain type of weapon, such as longsword, shortsword, trident, etc. Whichever you choose when you first manifest it. The size changes based on your own size, which may have variable scaling.