Poised Bearing (Armor Mastery, Combat)Source Armor Master's Handbook pg. 17
Your armor’s weight gives you stable footing.
Prerequisites: Base attack bonus +6 or fighter level 4th, armor training class feature, proficiency with medium or heavy armor.
Benefit: You treat your size as one category larger for the purposes of determining the size of creatures against which you can attempt bull rush, dragAPG, overrun, and trip combat maneuvers.Source Armor Master's Handbook pg. 21 (Amazon)
You can spend 3 stamina points as a swift action to have this feat’s effect on your size for the purposes of specific combat maneuvers also determine what size a creature must be to perform the combat maneuvers against you. (See page 17.)