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
http://svnbook.red-bean.com/en/1.4/svn.reposadmin.planning.html
http://svnbook.red-bean.com/en/1.4/svn.reposadmin.maint.html#svn.reposadmin.maint.diskspace
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.