From my brief time at the bike collective it seems 26" tires are the defacto, along with big box mtbs. Grip shifted 6 or 7 speed freewheels.
If I had the choice it would be a single speed rigid mtb with 2" 26" wheels, and a single speed setup that would allow me to pedal at an 80 rpm cadence at 12MPH, like a 42x23. That's 3x walking speed with a low enough gear to comfortably get up alot of hills. It also allows the use of a retro fit config from the existing triple and freewheel it probably already has. I'd try and splurge on alloy hubs instead of the usual stamped rear hub. A rack mount would be nice along with a backpack and bike chain/lock.
The SRAM and Shimano 6/7 speed grip shift systems are surprisingly robust though, and easily tuned.