How do I increase my managed DB's disk without increasing the other resources?

I logged into the Digital Ocean console today to find that a notification saying that one of my managed databases is running low on disk space.

I tried to go increase the disk size, but the only option I can find that will do that is the buy more expensive nodes. I’m on a 25GB node right now, and it seems to have more than enough RAM and CPU. The next size up will cost 2x the price and not even double my disk space (but it does double my CPU and RAM, neither of which I’m low on). So a few questions, which I’ve been unable to answer from reading the docs:

• How can I increase just the disk size?
• Where can I see how much of the disk is used at any given time, or do I just have to wait for a notification via the DO console? I would really like to integrate this kind of thing into existing monitoring solutions so detection and alerting can occur via the proper channels, instead of hoping someone signs into the console in time.
• Can I see the usage of the other resources? I said I have “more than enough RAM and CPU” but in reality I have no idea what kind of usage the DB nodes are seeing, and so I have no way to verify that. I just haven’t seen any reason to suspect they’re not.