If I wanted to make a worldspace that would be the equivalent of 100sq miles, what properties would I put in the boxes while creating the worldspace?

The Worldspace dialogue doesn't control the size of the worldspace. The x,y coordinates refer to the in-game map. The size of the worldspace is controlled by the http://cs.elderscrolls.com/constwiki/index.php/Heightmap_Editing. Each quad is a little over http://cs.elderscrolls.com/constwiki/index.php/Oblivion_Units in length, so you would need about 10 quads to create a world space this big. Memory limitations on the CS mean you would probably have to build it up in successive stages, creating a portion of the heightmap, converting it to an esm, expanding it in a plugin, merging it, etc., until you get the size you want.

Cool, thanks. I always thought those x,y coordinate things in the world creation thing would limit how big you could make your map.