Write File Activity Size Limit

I'm in the process of implementing a workflow that allows users to upload files to our server and create links in a SQL table to that file. I have used the SimpleOpenFileDialogTask form the Code Gallery and have it all working, however it seems to fall down when the files are too big. I have successfully tested it with files (JPG's & PDF's) that are up to 2Mb in size, however get a workflow error when trying to upload a 4Mb PDF.

I seems to be falling over at the Write File activity, so I just wanted to see if anyone else has seen this or if anyone knows of a size limitation on this activity? Nothing is mentioned in the help doco.

Thanks

Neil.