In general, a good rule of thumb is 1GB of database space per every 1000 users. For example, the recommended database size for 100,000 users is 100GB. Of course, the actual size could vary depending on frequency of use, size of attachments, content, images, links, etc.
For reference, here are database usage statistics for a server with 85,000 users containing historic data for 3+ years:
Data size: 100,214,240 KB (~ 100GB)
For more information on our system requirements: