How to optimize image sizes for product pages without cropping?

Hey everyone!

I recently posted asking about our theme, and I need some help again. I edited the theme to constrain our photos on the category pages to be the same height. Now, some of them are cropped like this:

(the book, the keychain, etc…)

I understand this is because some of our photos are too big. But I can’t figure out a standard size that looks good/uncropped on the product page AND doesn’t look low-quality once you click on the product.

Is there a standard size I should try or anything else I could do? I’m OK with resizing all my photos if I have to, I just need to know what would work well for both. I feel like when I get the thumbnail looking OK, the media once you click on the product is too small.

Here’s a link to the category shown above if that helps.

Thank you!

Sara @ Surfers Warehouse