Can you use a percentage?
Its easier to take an image - say 1400px 1200px (px is pixels) and divide both numbers by 2. 3. 4 or whatever. In this case: width=1400px height=1200px becomes width=700px height=600px when divided by 2. Then hit the preview button and see if that’s the size you wish - remember to always divide both height and width by the same number else the image will be wonky.
One of the best ways to find an image is to search for the topic, subject and then click Images. Find the one you want, shift click or whatever brings up an alternative menu on your system and select Image Info (the images should have sizing as you scroll over them). That brings up the image and its address along with the size, usually large images will have the small size in parenthesis.
hope that helps