The Filemaker Pro 6 database file takes up 2 GB on my local hard drive.
I believe FMP 6 files have a maximum file size of 2GB. This would be consistant with the error message: "...the maximum size for this file has been reached."
You'll need to either keep your individual file sizes beneath this limit or upgrade to a newer version of filemaker that supports larger file sizes.
FM6 had a limit of 2GB per file.
Edit: Didnt see Phil's response.