It really is just a preference. For me, the larger sized razors usually have more heft to them and makes it easier to keep the pressure ultra-light. With the smaller, lighter razors, I find myself applying too much pressure. Also, the larger razors hold more shave cream so there's less rinsing than with a larger razor.

Other than that, I don't think the size has as big of an influence on the shave as, say, the grind and honing.

As for the small areas that are hard to reach with a larger razor - use different parts of the edge until you find one that works right. The toe can get under the nose, and I use the heel for around the corners of my mouth. Just be sure to keep your eye on the whole blade no matter what part is touching your skin.