One thing I'll say from experience is that it takes a lot for size alone to become a major factor; if you allocate sufficient resources, the db can get pretty big without a problem.

To put things in perspective, our main repository has over 50 million documents, 21.5TB of volume space, and a database size of over 600GB.

Even with this much data, a broad search across all folders for a specific metadata field value still only takes milliseconds to complete.

Now we do have multiple repositories, but the separation was only made because the repositories play completely different roles within our organization.

Additionally, even if you create a separate repository, you probably wouldn't be creating a separate Laserfiche Server and that handles a lot of the workload anyway.

The bigger question is probably going to be how big of a difference there is in how the repositories are utilized, and what type and amount of activity each group is going to bring into the mix (i.e., text searches, scanning, etc.).

In my experience, what your users are doing has much more of an impact on performance than the actual size of the repository, and even then it depends on what kind of resources you allocate.