how much data can a 5MB SQL database store?
A little known Heroku secret is the 5 MB is not a hard limit. You can go over it by quite a lot (by 5-10x) before Heroku will get unhappy with you.
In any case, you would be amazed how far 5 MB can get you, particularly if you are creating the data by hand. For instance, if you are storing blog posts - and let's say 20% is overhead (metadata for each blog post), you would need to write 4,000,000 characters of text before your database hit the 5 MB mark, which is more than the contents of War and Peace.
So it won't be enough if you're storing entire books in your database, but if you're writing your next novel in it, you should be good for quite a while.
The formula is relatively simple to figure out, at least in vague terms. Basically, sum the size of each column, and then multiply that result by the number of rows. While this is a small overhead involved, it's probably small enough to be ignored.