You can use any MediaCar skin with Road Runner -- the older skins have bigger buttons in general (because there are fewer options). The Alpine skin has fairly big buttons in general.. The original Brushmetal skins do too..
To make/modify skins you just need a skin editor tool on my site.. it is open source too. It lets you add/change objects with ease.. all you need is the images .. you can get original PSDs at febsperanza's site (link on my site).. then modify to your liking. MediaEngine has pretty big buttons (uncommon to most front ends), you can make buttons of any size in RR.. but since you'll probably run out of space, you'll need to make some "popup" menus to access some features (MediaEngine basically always shows popups -- they just don't look like menus).