I actually like 49 keys...
25 keys is only for travel, bass lines and melodies
37 keys is barely two hands, but restricted
49 keys gives decent range while giving space on a desk.. the perfect all rounder imo
61 keys is most common, but its long and once on a desk, leaves little room for other stuff (like drum machines etc)
76 keys is perfect for piano, but rare as a controller
88 keys is just way too large and a good weighted one is the same price as a stage piano
and I have all except 88..lol