In Windows XP, if you hover your mouse over the file name, you will get a pop-up tooltip which shows the dimensions. Or if you have the task pane shown in Windows Explorer, you can click on the image and it will show under "details."
Or you can right click on the image, choose Properties, then click the "Summary" tab and it will show you the width and height there.
I'm sure there are similar functions in the Macintosh OS. (Mac users, please post your suggestions in the forum.)