Every element you add to a Boom Card has to be downloaded and rendered on a student's device. Card sizes are restricted. Most people will never come close to the limit.
There are some poor design choices that can bloat a card quickly. If you clone images on your card dozens or hundreds of times, each image must store lots of information about size/position/scale and the url to the data.
If you want 2 dozen "stars" in your "sky"... or a field full of pumpkins, it is much more efficient to make a single background image instead of dozens of individual images.
Card size is indicated by a status bar in the Editor.