I don't think this is a matter of mathematics at all! If you could play on the row then you are reliant on balance within the tune to keep your bellows in a comfortable central position. It was ever thus ...
If you play the Continental style then most of the musical phrases are on the pulll, particularly if you are not playing in the home key of your instrument. The main issue in that situation is the quality of your air valve, and ability to collapse over-expanded bellows … fairly quickly
Yes "size matters", but this is very much more the case in cross-rowing style where ... you can run out of left arm in :-\some tunes
Worse if you are improvising
As over we back to "it depends how you play" but I would recommend good quality, capacious bellows to any melodeon player investing in a new box.
He/she should also looks assiduously to the quality of the air valve. In my opinion the most important button, on any button box