Property Size is not available for database
I have resolved this by restarting the SQL Server service, unfortunately/fortunately.

My initial idea was to detach/attach the database. Not that I thought this would actually fix the problem, I just read that in a forum somewhere. This had no effect. 

I regrettably did not do what  PJ Mahoney suggested re: running a trace. I hope I see this issue again so I can find out if a trace reveals anything. Thanks for the suggestion.

At least answering my own question means I don't have to see another suggestion about changing the database owner ;P

Thanks