Hello Ann,
At the moment there is no way to determine the size of a shared (or personal) deck on the phone.
However, at least for shared decks, the information is available. I am not sure if this includes the media,
which is probably what would take most of the download bandwidth for some of the decks.
This sounds something that is reasonable to show in the list of decks before proceeding to download.
I am not sure how complex it will be, but I will try to look into it when I get a chance.
I will also file an issue for this feature request, so it does not get lost.