After using smaller tiles for a week (samsung ativ s with reg hack for smaller tiles) , I can say I dont get any issue with hitting the wrong icon.
Apps I use when in my car are set to medium (like swipe tunes and bluetooth shortcut), so they are easy to hit when driving.
The only gripe I have with the small size tile is its impossible to read live updating text on them. So I just use small for apps without live text (like sky news, webpages, settings etc).
Msoft should release this hack as a operating system setting in 8.1 so people can choose to fit more on screen, if they wish to sacrifice tile size (This shouldnt affect developers as it seems to be a rendering setting rather than actually changing the tiles size, as all tiles I have running are rendering just fine).
With this hack I finally I dont have to scroll up or down to launch the apps I use on a daily basis as it all fits on the screen now.