There's a couple of things you'll want to take into account.

First, you need to add folders into the mix. Each folder has a row in the database that is in fact in the same table as the documents. Documents are just nodes that can't have children. Any shortcuts are also going to have a row. Electronic documents are going to generate rows in the Docs (pages) table. I don't know what the rules are. It seems to create multiple pages, but I don't know what the split criteria is.

Next, you'll need to consider metadata. The size of the metadata depends on how many fields that you have in the templates.

In our repository we've got about 1.6 million Tocs (that's the table that holds documents, folders, and shortcuts) all with templates and varying numbers of fields. The repository database is approximately 11.4GB. Unless I messed up my math (hey, it's Friday) that equates to about 10.7KB per Toc and in the end a little over 93,000 documents per GB.

This is all back of the napkin stuff, so don't treat it as writ.