volume size to support permanent storage of Numbers File

I have a Numbers file that I store in an encrypted disk image. Every time I exit, after making a change to the file I get the following message - "The document “Words handheld.numbers” is on a volume that is too small to support permanent version storage."

The Numbers file is 250 KB and the volume or disk image has 448.9 MB free. How big does the volume need to be to support permanent version storage?

Running Number 3.6.2 on a late 2013 27 inch iMac with OS X El Capitan 10.11.6