RPG Maker 2003 does not support single-sprite set importation like later versions of the engine do. Every type of graphical resource needs to have a certain very specific set of dimensions in pixels or else it won't work. All the specific parameters are in the Help file included with the engine.
My advice would be to export an existing CharSet and use that as a template, without changing the dimensions of the image itself at all (no, you can't just crop out the other sprite sheets). Also, if you don't have it already, make sure to pick up Paint.net so you can convert the image into an 8-bit PNG file, which is the optimal image format for importing sprites to RPG Makers 2000 and 2003.