Many image software packages can tell you how many pixels your photos is. If you don't want to mess with trying to figure that out, you can use this page as a guide.
This is a picture 420 pixels wide.
If you can open one of your own pictures and compare it on your computer screen
you can kind of judge how wide your picture is.
This is a small picture stretched to 420 pixels wide. You can see why you should upload pictures that are at least 420 pixels wide.
