The first thing to consider is that "size" refers to two different things—one is the actual dimensions of the image (eg. 2x2 inches, or 540x540 pixels), and the other is the file size (eg. 1mb).
Product dimensions are not critical, the uploader will re-size the images so they will never look distorted. Ideally your image dimensions are less than 480x480 pixels and square.
Product images have a maximum size of 200kb, but in general should be 50kb - 100kb.
If you want to reduce the size of the images, you can do so using Google Photos, which is free and easy to use. Here is a tutorial on How to adjust, crop, or rotate a photo on your mobile device or computer.
Reducing Image Size
If you need to easily scale images down to fit the above requirements, we recommend PicMonkey.