I owned Speeds and when I did, I ran a 55 ring up front on a double crank. I don't remember the little ring size or the largest rear cog but I was just outside of the "wrap" spec and it worked fine.

My current machine uses a 700 rear wheel with a 50/36 up front and an 11x30 rear.

And, before anyone asks, my average speeds are identical. To be honest, I like the little rear wheel better because it accelerates from a stop like a jack rabbit. On the other hand, the bigger rear wheel trikes do ride a little softer without suspension. As for stability, it's more a function of design than wheel size. My Speeds cornered like they were on rails.

For the record, I've owned 2 Speeds, 2 700s, a VTX, a Vortex, an Expedition, a Windcheetah, and an older Road.