The same game I mentioned earlier. My son rolled a crit hit and did 2 damage plus 1 for the crit hit. How is knock back determined, is it 6? Is it always on double number?

You take knockback equal to the amount of damage dealt. If you take 4 damage, you get knocked back 4 squares. The damage is determined by the fact of whether or not you have a damage reducer upon falling off the building or into blocking terrain. Knockback damager occurs seperately from the damage that caused the knockback, so you re-apply any damage reducers to the knocked back character should they be present when figuring in knockback damage.

If you are using force blast, you roll a single 6 sided die and knock back the target character equal to what shows on the rolled die. Hope this has helped.

And yes. Any successful attack roll of doubles will generate knock back.

From p. 9 of the 2010 Core Rulebook:
Small correction...you take knockback equal to the amount of damage taken.

From p. 9 of the 2010 Core Rulebook:

That's what I meant.