How to quickly know the dimensions of an image?
Get Info
You can get the resolution from Get Info menu. Select the image and press ⌘ CMD+i, or right click on that image and choose Get Info menu item.

Image from cultofmac.com
QuickLook plugin
If you weren't on 10.9 - this lightweight QuickLook plugin called qlImageSize which can be found on GitHub lets You check the image size on the quicklook window title. I don't have any that work on 10.9, but perhaps this will help you if you wish to search for a similar one or report 10.9 compatibility to that project for improvement.