At what setting?
The thing is that while the original photos have a cumulative size, the edited ones don't. They are all virtual, and only become files when exported, and the size of those depends on the export seting you choose - Tiff/Jpeg/Png and the compression levels etc.
In iPhoto 08 this was not the case, as there was an actual modified file.
But the event does not really have a size - you can export the photos and make the size pretty much what you want - while it is in iPhoto it is an event
I guess that iPhoto could report the size of the original photos as imported - or the size of the modified photos if exported as JPEGs - or the size of the modified photos if exported with a maximum dimension of 1080 - but the event simply is photos and does not have a "size" until you export it
Obviously you want to know the size but the question was
what is your puprose for knowing the size?
WIth that information maybe there is a way to get you what you want
But the basic answer is simply that an event does not have a size - an event is a collection of photos and each photo has either two or three versions in the iPhoto library and each photo can be exported for outside use in several formats and at any size