Billboards have their geometry generated every frame depending on where the camera is. It is most beneficial for all billboards in a set to be identically sized since the engine can take advantage of this and save some calculations - useful when you have sets of hundreds of billboards as is possible with special effects. You can deviate from this if you wish (example: a smoke effect would probably have smoke puffs expanding as they rise, so each billboard will legitimately have it's own size) but be aware the extra overhead this brings and try to avoid it if you can.
Billboards are just the mechanism for rendering a range of effects such as particles. It is other classes which use billboards to create their individual effects, so the methods here are quite generic.