(In Windows flavored OS's)
Right click on the picture, select Save Picture As.
Go to the place where you saved it, and right click on it and select Properties. Under the Details tab, it should give you the dimensions & resolution. The higher the dimensions and resolution, the better (you'll get less pixelation as you increase the size).
For example, I did a save as from the posted pic (not enlarged, but straight out of your post) and the Horizontal and Vertical resolutions are 96dpi (dots per inch) with a resolution of 178x180.
*Edit*
If you're using Windows and use the default viewer for pictures, you can use your mouse button to scroll the size. As you make the picture larger, you'll notice blurring of edges and blockiness. You'll want to find a picture that's got enough information that you can get it to the size you're looking for on screen without this happening.