DR does not come into play with Stand Still, nor does, in fact, any immunity or resistance to the damage that the creature possesses. The damage roll is simply the result on the damage dice, plus any bonuses you might have. Note that Stand Still requires that you forgo an Attack of Opportunity and is not an additional effect on AOOs, Knock-Down gives you an additional trip attempt and not an AOO, and Improved Trip must replace an attack and does not generate an extra attack with knockdown. I am fairly certain you cannot use Stand Still and Knock-Down for the same attack, nor make a Trip Attempt with your Stand Still attack. You can, however, perform a Trip Attempt off of an AOO without using Knock-Down and replace the generated attack with a Stand Still attempt (as per the wording of improved trip, the melee attack is as if you hadn't used a trip attack). And of course, you could always use Stand Still with the AOO that they provoke by standing up.
My preferred feat for lockdown is Large and in Charge from Draconomicon. It requires 10' reach and large size, but it IS an additional effect added to an attack of opportunity, so it works with Knock-Down. Instead of rolling damage and them making a reflex save, you make an opposed strength check, where you get +4 for each size category larger than them you are, and +1 for every 5 points of damage you dealt with the AOO. They receive no bonus other than their strength no matter how large they are. Like Standstill, it also prevents them from moving any farther, and pushes them back to the square they just left.
As for getting your size up, there is always Giant Size, a Wu Jen spell that is also on the Hero Domain. It sets your size based on the spell's caster level, so it doesn't matter what your starting size is. Unfortunately it's range is personal, so you'd have to try your luck at buying a custom item of Giant Size, or get a party member to craft one.