Minimum volume size to enable versioning

I have certain documents stored on a password protected disk image stored on Dropbox. Since the documents are small text documents, I made the image small (100 MB). But now I'm getting the following message when I save a document: > The document “foo.tex” is on a volume that is too small to support > permanent version storage. > > You will not be able to access older versions of this document once > you close it. I have another 2 GB image which doesn't give rise to this warning. So my question is, what's the minimum volume size I should use to avoid this warning?