I'm not sure about Maiden's management knowing the stage size of every venue they play before hand unless it's something they ask when booking the venue. Still, it's really impossible to know until they set up for the venue if everything can fit or not. I'm sure there's other factor's than just the size of the stage. I think how much storeage room in the back and sides of the stage would play a big part in it as well.
Maiden obviously knew that not everywhere would be able to fit their whole production. They brought a banner to hang in the back during 7th Son in place of the Eddie when they couldn't fit it.