*bow-bow-bow-* to your awesomeness! This is exactly what I was looking for!
You can attempt to trip your opponent in place of a melee attack. You can only trip an opponent who is no more than one size category larger than you. If you do not have the Improved Trip feat, or a similar ability, initiating a trip provokes an attack of opportunity from the target of your maneuver.
If your attack exceeds the target's CMD, the target is knocked prone. If your attack fails by 10 or more, you are knocked prone instead. If the target has more than two legs, add +2 to the DC of the combat maneuver attack roll for each additional leg it has. Some creatures—such as oozes, creatures without legs, and flying creatures—cannot be tripped.
So 4th level, the halfling is considered medium, so trip large size creature?
so 8th level, halfling can trip huge size creature?
so 12th level, can trip what size?
Underfoot Trip (Ex)
At 1st level, an underfoot adept learns a number of maneuvers and grabs that can cause even the largest opponents to stumble and fall. He gains Improved Trip as a bonus feat, even if he does not meet the requirements. At 4th level, and every four levels thereafter, he acts as if he is one size larger for the purposes of determining the maximum size of creatures he can trip and when determining his CMB and CMD for purposes of a trip combat maneuver.