61 note keyboards are always either too much or too little for me. If the synth is primarily a lead or bass machine, then 49 keys is OK. If the synth has enough polyphony to be played like an organ (or piano), then I’d think 73 keys is about optimum. Like the old Akai AX73 - perfect keyboard length, but unfortunately terrible action and no aftertouch. 76 keys would be OK, too, I guess. Gotta have aftertouch, though.
And as far as 88 notes … I’ve never needed that many, and I probably never will.