Heroku provide instructions for provisioning a new postgresql database, which involves a command of the form
heroku addons:create heroku-postgresql:standard-0
where the text after postgresql: is a key to the level of database. Presumably standard-0 is the lowest level of standard. It took me about ten minutes, using the web interface to be reasonably sure that hobby-basic is the key for the highest level of basic. Can someone please provide a table that gives the key for each of the database levels so we do not have to guess?