Elias here with Shopify Support. Thank you for reaching out.
I understand you're looking to make all your product images the same size on your collection pages. While there are several different methods that you can use to make all your images the same height and width, though I do feel that taking the photos using the same aspect ratio and then re-uploading them would yield the best results. However, you can also edit the images using the image editor directly on Shopify, or you can use a photo editing program - such as Adobe Photoshop or PIXLR (free).
Alternatively, there's also third-party apps, like Photo Resize, which can help you resize all the product images automatically. If you need any help with this app, the app developer is always available to assist you. You can reach out to the developer directly through the contact information found in the Support section of their app's page.
I'd love to know if any of these options can be a solution for you. Keep me posted!
Thank you for following up!
It is possible to change the aspect ratio for your theme through CSS. However, as outlined on our design policy, this isn't something that's supported by our Theme Support team. Nevertheless, if you'd like to explore this option, you can hire a developer to assist you with this. If you don't have a developer, you can consider finding one from Shopify Experts. Shopify Experts is our directory of certified developers who are experienced with custom coding and they'll be able to help you create a custom solution for your store.
As suggested by @MS-WEB-DESIGNER, resizing the images by retaking the pictures and uploading them again would likely yield the best results. Would this be something that you'd consider doing?