For people like me, it is the size, but in the two-dimensional sense. As an artist, I have been most comfortable painting on canvases that are 20"x24" or more, like 40"x40" and 40"x48". It is an accepted fact that a larger surface creates a greater impact.
But it does not mean that one paints a better painting on a larger canvas. A good artist paints equally good pictures on both sizes. But the point is about comfort. If an artist ios comfortable with a larger canvas, then why not?
I have been trying to make smaller paintings too. And the greatest advantage, I feel in that, is the lesser paint and time it takes. A smaller painting gets done faster, so my results are there to see quicker than it would be normally if I do a large canvas. And I think that is not too little an advantage.
And then, one can do a lot of practice in smaller formats. Like with charcoal, ink and crayons. As the days pass, I discover more advantages.
It will be some time before I find out the exact size that would be right for me. But meanwhile, I would continue to work on both large and small canvases. There are joys in both.