The bonus links should be approximately 15x15. This is a good question, I'll add this to the help documentation.
WarLight uses the svg format for maps. I haven't used Illustrator, but a quick search pulled up this page for help:
You probably want to uncheck the "Preserve Illustrator Editing Capabilities" option, since WarLight won't use the illustrator editing capabilities and they will likely increase the SVG filesize.
Once you have the svg, you can follow the instructions under Help to upload it.