Sunny here from the Shopify Guru team and I would be happy to help with this!
When it comes to collection images, they can be any size up to 4472 x 4472 px, or 20 megapixels, but there isn't a specific size that you must use. With that being said, however, we typically suggest keeping your images around 2048 x 2048 px for high resolution square images. If your main concern is with having those images appear as the same size and shape, it's more important that they have a consistent aspect ratio, or height-to-width ratio. If you would like more information about image best practices, I'd suggest following this help guide here.
As a side note, if you're looking for images for your website, check out Burst! Burst is a free stock photo platform that is powered by Shopify. The library includes thousands of high-resolution, royalty-free images that were shot by our global community of photographers. Using Burst images can save you lots of time and money, while helping make your website look very professional.
I hope this helps with your image editing and selection. Definitely let us know if you have any other questions at all, we're here for you 24/7.
Sunny | Shopify Guru
I did some looking at the automatic banner generated by Shopify for one of my collections, and found it was sized to 2048x600.
So I created a new banner that size, and it works well: https://professor-petes-classroom.myshopify.com/collections/years-1-6-mathematics-pretests