Due to an old ankle injury and multiple surgeries on same - I found out the hard way that I cannot rotate my left foot enough to get out of clipless.
I wanted the largest platform I could find - did a good bit of looking on forums and the internet.
Settled on Speedplay drilliums. Use a pair of Teva pinner shoes with them.
Haven't slipped off, they ride great for me - If you lift your foot somewhat on the upstroke, you would be surprised at how much you can pick up the cadence.
Note: at the time Speedplay did not publish width/length/height. I called them, no definitive answer by tech, so I had them get a pair out of the warehouse and measure.
They seem huge, and just right.
Contact surface is slightly concave -
3 9/16 " front to back
The above measurements are contact surface only, and are not overall - a little larger overall due to offsets of surface.
Pricey, but I am quite well pleased.