What are the restrictions on the length of a flatfile db? How are the number of entries on a flatfile page controlled?
You will only be limited by the amount of free-space on the servers hard drive and the file system used on the drive. For example if it's ext3 with a 4KiB block size then you'd be limited to 16TiB. When doing a database in a flat-file, you are the designer of how the database is formatted. Most people do entries by line, some do spaces. It will be up to you and your needs. Keep in mind though that for a larger database, any SQL server (e.g. MySQL, MSSQL, PostgreSQL, etc.) is going to give you much better performance.
Here is an example of using a flat-file as a database: http://www.designdetector.com/archives/04/10/FlatFileDatabaseDemo.php#demo