This is a difficult question as it has to do with available resources, values which are in flux as the machine is used. Here are my thoughts on database size, bearing in mind these are comfortable limits, so YMMV:
Size in gigabytes isn’t the critical number. If you check out File > Database Properties > … for a given database, the number of words / unique words are more critical. On a modern machine with 8GB RAM, a comfortable limit is 40,000,000 words and 4,000,000 unique words in a database. (Note: This does not scale in a linear way, so a machine with 16GB wouldn’t necessarily have a comfortable limit of 80,000,000 words / 8,000,888 unique words.) So text content in a database is far more important.
- If you have a database of images, it will have very few words but be large in gigabytes.
- If you have a database of emails, it will have many words, but may be smaller in gigabytes.
The second one may perform more poorly as the number of words increases beyond the comfortable limit.
Smaller, more focused databases will generally perform better, Sync faster, and be more data-safe in the event of a catastrophe (avoiding the “all your eggs in one basket” problem) . They also give you the opportunity to close unused databases when you’re not using them. This frees up resources, not only for DEVONthink, but the rest of the system. I personally advocate closing unused databases. There’s no real benefit to using machine resources on data that’s not in active use.
Some related thoughts on this are in the built-in Help > Documentation > Getting Started > Building A Database > Adding your files