I have tested more portable MIDI controllers than I can keep track of, and I will tell you right now: 37 keys is the ideal size. While Arturia's 25-key MiniLab MK3 is a solid controller that easily fits in a backpack, it feels a bit claustrophobic.
Source: [The Verge](https://www.theverge.com/tech/948024/arturia-minilab-37-midi-controller-review)