I wish to deploy my blog on AppHarbor. However the free size the database is 20 Megabytes. I'm not prepared to pay $10/month for that 10GB database. My real question is: would this be adequate for any simple blog with one publish per week?

I'm not likely to use any blogging engine. I'll write my very own engine with only posts within the database. I intend to use Disqus for comments.

That's fundamental math, really.

One publish per week of say 10kB per publish, will make you last 200 days. Mutatis mutandis:

          D bytes per database
---------------------------------------  = weeks per database
(N posts per week) x (S bytes per post)

It ought to be enough. Take into account that our blog (wordpress based) has almost 1000 posts and around 1000 comments. And also the database is under 5 Megabytes.

yes, 20MB of information is really a lot of information (whether it's text based).

If you are simply using text, no images or media, that needs to be fine. It requires lots of text to achieve 20MB...

Roughly it offers a superior 4,194,304 words to experience with (given 1 byte per character, average of 5 figures per word)

for images u may use something similar to photobucket, where only links should be copied and pasted.... 20 Megabytes has become a quantity! I'd reckon u begin with 1 Megabytes from the weekly basis after which go onto 20 Megabytes, helps you to save a little of $$