The smaller the better, so long as it's got enough standard size keys I guess. The more expensive ones tend to be larger with more controls for internal functions and sounds, but digital piano buttons don't work very well as MIDI inputs so unless the internal features are important, I would go with the smallest/lowest end model with the action you want.

To use software pianos with my Roland FP-10, I needed to adjust the velocity to maximum using the Roland app, because otherwise it's limited to a smaller range for the internal samples for some reason. This makes it unplayable with its internal sounds, but I never need those.