mongodb/mongo-csharp-driver — comment on commit e5d8afe6092e78074ee74b6c9a70152f72a5438e
File:  line 0
By rstam:

It is a count call to the database. Size is the same as Count but it respects skip and limit. It's just a nasty naming convention inherited from the mongo shell (nasty in the sense that Size and Count sound like synonyms so it is unfortunate that they do slightly different things). I should have just made Count in the C# driver work like size in the mongo shell and not bothered to implement the shell's version of count... :(