The 16Mb limit on document size seems very arbitrary indeed, given that Mongo is capable of storing documents with much larger (about 536.7Mb, I think).

Could this value be configurable?

I am not thinking of situation with large binary data, GridFS can handle that but scenarios where there are growing documents (such as the scenario I'm looking at in work).

If a 16Mb hard limit was reached, and the database simply refused to carry on letting the document grow, then this would cause a huge problem.