Little bit of a grind but I’d take all of your product photos back to Publisher or something and re-size them to all be the same starting size.
Bit of a long-way-round solution but in the past when I’m not sure what size looks best I literally upload a bunch of rectangle PNG’s and use that the determine which size will look best in a certain page/carousel. Then I’ll re-size a bunch of photos just to that size.
The less work you make Shopify do re-sizing and fitting your images for you, the more cohesive it’ll look over all.