A 71KB flat file database is pretty small by today's standards. If it's a single-user database that you aren't performing complex queries against, you are potentially fine up through several megabytes.

But there are reasons beside size to consider moving to an RDBMS. For one, you get the benefits of indexing (fast record lookup) and SQL to allow you to express complex queries. Perhaps not an issue if you're never going to grow past a single table, but something you'll need as you split into multiple tables. Next up is concurrent access control, which you can do by hand (via flock()) for a single table database, but which is a major nuisance once you get past one table. And then there's the ability to roll back a transaction. Again, not a problem for single table flat file database, but a big win when you're doing something involving multiple people doing work simultaneously against multiple tables.

There are also costs: RDBMSs require a bit of care and feeding that flatfiles don't. This varies by RDBMS; MySQL isn't too bad.