Usually when you have this question “how do I make my images the same size”, what you really need to modify is the aspect ratio (the length/width ratio).
What aspect ratio should I use?
You should use the aspect ratio that most of your images are already in. If most of your images are in 1:1 square ratio. Then you should resize the non-square images into square. Sometimes, it is difficult to have one ratio for all your images. For example, you may have clothing collection in 2:3 ratio because they are usually standing models and images are mostly portrait. But you have bags in square ratio as well. Then it is okay to have 2:3 for clothing and 1:1 for bags. But generally, you should keep as fewer number of different ratios as possible.
How should I standardize the aspect ratio of my images?
There are two ways: expand or crop.
Cropping is ideal if you have enough margin on your images. So cropping will not cut into the main object.
If cropping is not an option. You should expand. Here I'm going to show you the steps for expanding to a ratio because in most cases, this is the best option.
Select "Resize" from left menu.
In the filter you can define "all products" or "a product collection".
Select "resize canvas of the image to a defined aspect ratio".
Input the ratio. Here I want 2:3.
The margin colour should be same as your website background colour.
Here's an example of a merchant's collection page before and after the resize: