I would like to install a Wordpress multisite, than would increase in scale everytime I create a new user. The scaling does not have to be automatic. I'm thinking that I would like to linearly increase things like RAM, CPUs, bandwidth and SSD size by the same amount for each user. The solution should also be down-scalable. Are there any out of the bag solutions for this?
The DigitalOcean API would be your best bet if you want to add some level of automation, though it's meant to be used programmatically, meaning you would need to interact with it from the CLI or using a programming language.
DigitalOcean provides the ability to scale in two ways when it comes to CPU, RAM, and Disk
1). Scale CPU & RAM Only -- You can scale up or down as needed.
2). Scale CPU, RAM, and Disk -- You can not scale down once disk is resized.
With that in mind, if you need to be able to scale the disk more easily, I would recommend using the Block Storage service. It's far easier to work with when it comes to disk scaling and doesn't require that you shut down the droplet to do the scaling (as would be the case for #2).