The space required for Mac OS 9 may be higher than the Installer indicates if the disk you're installing on is formatted as a Mac OS Standard format (HFS) volume. To reduce the chance of the installation not completing, you should free up an additional 25 megabytes of disk space.
If your disk or volume is larger than 1 gigabyte, you should consider backing up the files and programs stored on it, and reformatting it as a Mac OS Extended format disk or volume using the Drive Setup utility. After reformatting you can restore your files and programs from you back up. This will provide considerable additional free space for future use.