Personally, I'd feel that just 3 sizes is a bit restrictive.
I find scrolling the start screen to be annoying, but to avoid that I end up with a screen that looks like a flat version of the iPhone.
A tile that is 2 x small tiles wide, and a tile that is 2 x small tiles high, might be useful.
I personally thing the 3 sizes are enough but more choice is always good and perhaps they will think of implementing some new sizes. However, if you make double high or double wide "small" tiles wont that just take up more space and you will still end up scrolling?
Edit: I would like to see a similar "grouping" feature to W8 wish a space separating groups of tiles, that'd be nice.