Named in the best traditions of unix (no wonder I can never remember the damned name), rdjpgcom is a utility which can tell you the width and the height of the JPG file!

From the man entry:

rdjpgcom reads the named JPEG/JFIF file, or the standard input if no file is named, and prints any text comments found in the file on the standard output.

Comes handy when you don’t have access to ImageMagicks excellent identify.