[[[In my experience of pub session playing, having a smaller box is an absolute must]]] Fervent agreement. I am loving small CBAs and small PAs for seshes . . . and gigs, and practice, and travel, and . . .
I loathe the scant bass systems in melodeons. If you are going to play a melodeon smoothly "across the rows" as in, say, B/C Irish accordions when playing in D, G, A, E, and more, plus their relative minors . . . why in the world would you put up with the inconveniences and expressive frustrations of a melodeon rather than a small, compact CBA or PA where you have all notes in both directions, and on the left side a 72 bass, a 60 bass, a 48 bass, or heck, a 40 bass or a 32 bass, gives you exponentially more bass options? I just see no point in playing bisonorics unless you are mad for the "one-row," old-school push-pull sound, which I enjoy only for super-simple dance tunes like polkas.
But having said that, I agree one doesn't need the full 120 or even 96 or 72 to play folk music brilliantly on a small unisonoric. I do like having a chord row for each of the 12 tones, but don't need diminished or even seventh chords. Love a 60-bass or a 72-bass, but find 48, 40, and 32 work great.
36 would really do it: 12 root notes, 12 counter-basses, and one chord button, either with no thirds at all, or with a stop to remove the thirds. Unlike Craptiger, I do like to have Bb, Eb, and Ab--I've seen a 60-bass in a 10X6 config that adds to the classic 8X6 48-bass layout by giving you your "B" at one end and your "Ab" at the other, and really like that. Any C#s or F#s I'd ever need I can get fine using single-note counter-basses and root notes.