Don't forget "angle of attack", going in straight up and down is an invitation to be countered. As is stiffing uke with your tsurite (one of my students got spectacularly countered with Tomoe Nage upon attempting Ouchi Gari with a stiff/straight lapel grip.
I have more questions for BKR et al regarding Ouchi gari.
In particular, tsukiri. I've been told/heard 'be chest-to-chest' dozens of times. Yet with cross-stepping, my torso rotates some. Do I compensate for that by drawing uke's sleeve down (which also seems to rotate his torso into mine)?
1.) It's mostly a matter of training yourself to keep mostly forward.
a.) I do a drill against a wall in which I do the cross step/T-Step/or straight in step with the left leg (for RH throw) in which I (slowly at first) concentrate on keeping my chest/waist facing the wall. It's a bit of a twist, and you can hold it statically to stretch as well if you like. You have to adjust your distance so you are not too close to the wall. You can also practice the cutting motion to the side and then back in a circle.
b.) Working the drill I mentioned earlier, where you pul an uke who is in a wide shizenhontai forward with tsugi ashi while he/she resists to the rear, again concentrating on getting chest/waist forward. You can move on to uke being in migi or hidari shizentai (for a righty or lefty respectively). This drill works for Kouchi Gari as well.
2.) Its' also a matter of flexibility in your hips/waist/obliques/lower back. You can work on normal stretching stuff for that. I found that the various seated twists used in Hatha Yoga work pretty well, although I am sure you can do your own research on specific stretching exercises.
I have found that drawing uke sleeve back to my waist works well. i was originally taught to make an "A" pulling both hands down. If you watch most Japanese (higher level) doing ouchi gari, they seem to pull uke hand to their waist, which looks to me like it "pins" it in place so uke cannot escape as easily.
One thing about Ouchi is that it helps to get a reaction of uke to his rear (somehow) before entering.
I always say in Judo, do the opposite thing first. So to throw uke to the rear, move him forward/sideways/circle first. Even in sideways movement some action to the front is good/necessary, same with circling. You can use a slight shift of your weight to your rear (uke front) along with subtle hand action (bending wrists towards yourself) to elicit a reaction of uke to his rear quadrant.
Well, most Judo throws hinge on getting below someone's center of gravity, so a taller person is at a de-facto disadvantage against a shorter. Of course, a discrepancy in skills can make up for that, but I am presenting things in a vacuum here. If you are taller, you are going to have to get lower and be more explosive against a shorter person than you would against someone your size. Careful about Osoto-gari, there's a big gap between how it is taught in most places and the way it is actually (successfully) executed against a resisting uke or in competition...