How it works
A square image crops the same way in every theme, so you avoid cut-off products on collection pages. 2048 by 2048 is the size Shopify recommends for zoom.
Use the same template for every product in a collection so the grid looks consistent, then add a second image per product on a different template.
The 2048 size is on the paid plan. The free plan renders 2000 by 2000, which also works well on Shopify.
Questions
Which size does Shopify want?
Square images up to 4472 pixels. 2048 x 2048 is a common choice.
Can I use the images commercially?
Yes.