My name is Alex, I'm a guru here at Shopify!
There isn't a set size for the images you use there - though we typically suggest 1024px by 1024px as a guideline for square images! If you're more concerned with having those images appear as the same size and shape, it's more important that they have a consistent consistent aspect ratio, or height-to-width ratio. There's more information on working with your images to get the best effect in our guide right over here!
Hope that's helped! Please feel free to give us a call or start a live chat at any time, we're open 24/7 for your convenience and always happy to assist!
Alex | Shopify Guru