I have 6,8,10,14 inch pads. Larger pads are nice because they can fit right in a snare stand. Smaller pads are nice because they are portable.
In reality you should be hitting in the middle of the drums unless you are intentionally going for a specific sound near the edge.
I actualy think a 4 inch pad would make you much more accurate. If you are missing the pad you need to work on accuracy. I don't think other than the fact it fits in the basket there are any benefits to a bigger pad.
My blue logix pad is large and has a rubber rim. So maybe if you wanted to work on rim shots that is another plus.
I like a bigger pad, a heavier one too, as it'll stay in place on a snare stand.
I've tried some smaller pads that were ok as they came with an appropriate stand.
I'm not much into pads these days, but again I don't always have a choice. The main on that I feel doesn't mess with my technique is the Xymox laminate type of pad. The traditional Remo pads are great, but they're just soooo loud. Might just as well play a drum as that one.
Choice of pad does to a certain extent depend on you and the type you want to practice.
I love the Vic Firth Heavy-Hitter StockPad. It does not describe how hard I hit, because I am not a heavy hitter. I don't really know if it is the best pad to practice on, but it feels and sounds so nice it makes me want to practice.