For me anythign would work as long as the keywidth is standard... Having smaller keys works contra for most beacuse o automation and my fingers finding the keys blindly.... with smaller keys it got me staring at the keys...

The smallest possible would be a 49 key with no side pannels, and just a minimalistic bar with buttons and a small screen.. battery operated, with some speakers in the back



And, yes we are all jealous about the mobillity of a guitar...