Open all the images in Photoshop
Check all the sizes, that's both the width and the height. There is no set image size, as you need to check all the images to see what you have and which is the best common size.
This is a long and involved step.
You can always make a large image smaller, but never a small image larger without pixelation (stretch and blur).
If some images are different in size to the others, then the following steps must be taken after you have decided what is going to be your ideal image size.
Set an ideal image size and Image Resize with the 'constrained proportions selector' on.
Re-size all the images to your set size.
However you may get some that are not exactly the size you need.
Then for these images you must use this option, by switching off the constrained proportions.
You can then individually change the sizes, but remember smaller adjustments not larger unless it is less than 10px, which shall not affect the overall image size balance.