With this i think you are concerned more about user experience than about
SEO
I personally do not hang around webpages where I have to scroll and scroll to find what I am looking for
Also people tend to move on quickly if they don't find the product they are looking for within so many seconds of landing on your page
So I would suggest a page length that involves little scrolling
And if your pages have to be of such length that you have products below the fold of the page, I would suggest you put your bestselling or most searched for products above the fold of the page