Actually you'd want to base it on the average filesize used per record. Easiest way is to take your current database and divide its size by the number of records.
For example, my database is 87KB, and I have 166 records. So, my average record size is roughly 524 bytes (about 1/2K).
Now, take the rough physical size of a meg, in bytes, and divide by the average record byte size. (1,024,000 / 524). This calculates the estimated number of records I can have in a 1MB file: 1,954 records.
Of course, the actual number will be different, because not all of my records are 524 bytes. But this gives you a good estimate.
--Lee