Find Wallpaper Size on a PC
Step 1: Minimize or close any open windows to get to your computer desktop.
Step 2: Right-click on the desktop and select "Personalize."
Step 3: Choose "Display Settings," or for Windows 7, "Adjust Screen Resolution."
Step 4: Check the current resolution for your computer monitor and then close the window.
Find Wallpaper Size on a Mac
Step 1: Click the Apple icon in the upper-left corner of the screen, and select “About This Mac.”
Step 2: In the window that pops up, click the “Displays” tab.
Step 3: On the next screen, you will see information about the display (or displays) you have built-in or attached to your Mac.