I have been playing a wide variety of different models for years, as I’ve been a melodica collector.
First, while different folks have different preferences, I would strongly recommend Yamaha or Suzuki (metal tray models) over any current Hohners. Around 2011, Hohner made a corporate decision to emphasize mass appeal over quality in its melodicas and to move production to China. The emphasis has been on snazzy cosmetics. My preference here is also based on having disassembled and tuned a bunch of melodicas, vintage as well as recent models.
Second, I’m just eyeballing right now, though I have sometimes measured keys, but there is little noticeable difference in length and width between the most common current Suzukis and Yamahas: Yamaha Pianica P37D (now E but that’s a different discussion, and I’ll only say that I think the P37D is the safest bet), P32D, Suzuki M-37C and M-32C. Any one of those four would be a good choice.
Finally, many melodica players say that it’s better to think of the melodica as a wind instrument than as a keyboard instrument, and better to hold it in your left hand with which you can move it around than to play it on a flat surface blowing through the tube. I agree with those opinions. Key size is, it seems to me, much less of an issue if you follow that train of thought. There are advantages to having smaller keys that can be more easily appreciated if that’s how you approach the melodica. Of course there are situations in which laying it flat and using the tube makes sense, for example if you’re playing a regular keyboard at the same time or intermittently.
As I say, not all experienced melodica players will agree with what I’m saying. I’m just speaking from my own experience. I personally enjoy playing smaller models such as the Yamaha P25F (whose keys are about the same as the bigger models) and also playing older models with tiny keys such as the earliest piano key style Hohners.