I like kreynolds version of the vorpal, a lot! Thanx kreynolds...I will have to say I need to incorporate this variant in the upcoming campaign at Christmas.
However, unless the vorpal weapon expands to indefinable lengths like an uber lightsaber, I will go with a reduced effect based on target creature size and vorpal weapon size.
Such as, if with a vorpal weapon you score a critical against a creature larger than the weapon size, reduce the force damage by x3 per size difference but to a minimum of at least x1 force damage. If wiht a weapon of sharpness you score a critical against a creature larger than the weapon size, reduce the force damage by x1 per size difference but to a minimum of at least x1 force damage.
Something like this, incorporating a force multiplier reduction:
...............................................Target Size & Force Multiplier
Weapons by Size.......Small......Medium...Large......Huge....Gargant.....Colossal
Kukri (tiny)
sharpness..................x3..........x2..........x1..........x1..........x1..........x1
vorpal.......................x7..........x4..........x1..........x1..........x1..........x1
Handaxe (small)
sharpness..................x4..........x3..........x2..........x1..........x1..........x1
vorpal.......................x10.........x7..........x4..........x1..........x1..........x1
Longsword (med.)
sharpness..................x4..........x4..........x3..........x2..........x1..........x1
vorpal.......................x10.........x10.........x7.........x4..........x1..........x1
Greatsword (large)
sharpness..................x4..........x4..........x4..........x3..........x2..........x1
vorpal.......................x10.........x10.........x10........x7..........x4..........x1
*Large slashing double weapons count as medium-size slashing weapon for purpose of force multiplier.
Sheesh, I wish I knew how to setup tables...

Hhhhmmm...maybe the market price should be adjusted for weapon size differences.