What are the best practices with respect to planning disk space for a Subversion repository that will host average development projects (i.e., with text files mostly and the odd binray here and there)?
If, for instance, my project takes up, say, 100Mb as my working copy, how much space should I reserve for the repository to be comfortable?
While there aren't any workable formulas to help you calculate estimated growth and disk usage, the following Subversion manual topics should help you gain a better understanding on the underlying data store implementation
The backup and migration of a repository to another disk is fairly easy, so you can have the option of moving over to a larger drive when disk space is really getting exhausted.
A factor of minor importance: You may also want to determine if your estimate against your local working copy is including all the intermediary build artefacts and svn client files; those files should not contribute to your 100MB project size. A fresh export from your repository should provide a more accurate figure of only the files under version control. But as mentioned by others, that is not as important as the number and size of changes/revisions the project has undergone.