I was test-uploading a large map I was working on, and it said it was too big, and gave me a number of pixels I must be under. How can you re-size your map with all the territories and the background image I used?

Just select everything and drag the corner to resize? Then reduce the canvas size, I think it's in File -> Document options on something along these lines in Inkscape.

Embedded images unfortunately won't work on Warlight anyway.

Instead of hand-dragging resize better choice is the change of the horizontal and vertical size numbers to keep the precise rates.

But your oversize is caused probably by the background image, that is invisible in the warlight and makes too much file size. If you dont want to remove, you can try to convert it to vector format.

What does converting to vector format mean? :)

The warlight can visualize only vector format(.svg) images. Any other (png,jpg,etc.) images are absolutely unnecessary and invisible in the map file and makes only oversize, like in your map. If you want anyway the picture in the map, you can convert it to vector format with a converter program, for example Vector Magic.