The shapes (circles for hotel conventions, stars for camping conventions) scale such that the area of any shape grows linearly with the attendance of a convention. A minor note: because this map is a Mercator projection, conventions closer to the Earth's poles (that is, further to the north in the Northern Hemisphere, and further to the south in the Southern Hemisphere) will have larger shapes than conventions of the same size closer to the equator. This is mostly noticeable in the largest conventions. I find it regrettable, but I do not (currently) know how to make this map be a different projection.
The shapes are colored according to the time of year the corresponding conventions happen (at least, the time of year of the expected next iteration of said convention, if a specific date has not yet been announced). Specifically, they are colored such that cons at very different times of the year "look" more different from each other; roughly, conventions that are happening imminently are green, conventions that just happened are yellow or yellow-green, conventions that are 8 months away are red, and conventions that are 4 months away are blue, with times in between being mixtures of these colors.
Ever wonder about the logistical needs of a convention from its host venue/city, furry or otherwise? I did! If you're curious, you ought to look at this report from Ontario's Ministry of Tourism, Culture, and Sport. While several years out of date, the beginning of section 5 ("Demand") and most of section 6 ("Key Success Factors") remain relevant (and are not completely irrelevant to non-North American audiences). If you yourself are interested about starting a furry convention, get in contact with the Furry Convention Leadership Roundtable. Though founded in the US, it is an international organization with a wealth of experience who may be of some assistance, and also who attempts to assist in avoiding geographic and temporal conflicts where possible.
Data is largely pulled from this source and the specific WikiFur pages for each convention.