You can use TortoiseSVN for this (cf. Troubleshooting Version Control and Team Server Issues | Mendix Documentation and System Requirements | Mendix Documentation) but at your own risk ofcourse :)
I hope this helps!
Rather than removing the revisions, maybe you can just start working on a branch line instead of the main line once the svn folder grows too large?
Thank you for your suggestions. It turns out that in this specific project, the userlib directory was unusually large (700 MB). The .svn directory (also about 700 MB) was huge because it was retaining revisions that included 3 large .jar files: aws-java-sdk-1.11.775.jar (159 MB), aws-java-sdk-1.11.775-javadoc.jar (215 MB), and aws-java-sdk-1.11.775-sources.jar (143 MB). I don’t know why those files were there because the project doesn’t appear to use them. It was probably left by a developer experimenting with the AWS SDK. Once I removed those three files the userlib shrunk by 500 MB and when I committed the change the .svn directory also shrunk by about 500 MB.