We recommend a database availability of 10 GB to start of with. The reason being, there are two major features - Advanced Event Viewer and Throttling Analyser, that collect all the data and stores them in the database. With your Data Purge policy set to default, and if you do not have too many BizTalk hosts and host instances, 10 GB should be more than sufficient. 

We also recommend you have memory of 8 GB or higher if you are installing BizTalk360 on a stand alone server.