Making an image bigger on screen

Hello,

How do you make an image bigger on the screen? Like when you crop or resize an image and it gets small and hard to see, how do you get closer to it? Not change the size of the actual image though, just how it appears on the screen.

I thought it was the + and - buttons on the side of the screen, but that isn't working. I'm using a PC if that makes a difference.

Thank you!