Is there a way to only use make a large monitor only use 20 inches?

If you want to see what something would look like on a 20" screen, it won't work due to pixel density. a 70" 1080p screen has the same number of pixels as a 20" 1080p screen, they are just bigger on the 70" so therefore if you want to see only 20" of a 70" screen, you would only get a resolution of like 600x400 or something.