Hi Ben,
The use case here is validation protecting the reporting tool from spam. The LRS is able to choose a limit on statement size to protect itself from spam and the reporting system should be able to do the same.

If we need a summary, it could simply be a list of statement ids. If we're going to allow tools just to get statement ids returned by a filter anyway, the reporting tool could make two queries and then compare the list to get the ids of any that were filtered out due to size, if required. 

Andrew