Projects dont't have a set size and therefore trying to get the size of one doesn't make any sense.
A Project is made of of many different parts, there are the originals of course then there are the versions of these originals which only exist as instructions of how to turn the original into the version and the metadata for these originals/versions. And then projects can contain sub-folders and albums both regular and smart.
As most of this data (except for the originals) is stored in database files trying to come ip with a size for anyone project is not possible.
You can of course add up the size of the originals in the project but even that isn't a good estimate as some originals might be referenced in which case they take up virtually no room in the project.