They "need" to be 16 by 16. Well, is says it's the standard, so it is possibility for exceptions I suppose. I mean, you can definitely have them bigger, and in other shapes then a Box. Not sure whether a smaller would be accepted. Can you provide a Preview map link or screenshot?
https://www.warlight.net/wiki/Creating_the_SVG#Bonus_Links
Edited 8/20/2015 15:51:48