Wondering how extensive it can be … I’m a note fiend (5-8 meetings a day) and usually keep notes on everything. 6 months in with Agenda and I’m wondering how well the application will support my usage over the next 2-3 years; will search, indexing, tagging still be quick & snappy?
I know it’s a dumb question but I also remember my Onenote notebooks approaching 1gb+ of data and starting to sludge a bit 
Things like search should stay quick, in the sense that we use a database index for that which will scale very well. We have some UI improvements planned for it too, but that is a separate thing.
Scaling should in general be fine as long as you don’t make enormous projects. If a project begins to get too big, make a new one. Agenda basically has separate documents for each project, so things are generally fine as long as one document doesn’t get huge.
Having many projects will mean an initial sync up takes longer, but once everything is up to date, it should be snappy.