Yeah, they are pretty tiny. Like the screen on the Korg monologue or minilogue. For 20 parameters plus waveform display it would definitely be a bit of a squeeze!
There are a couple of separate things here. There is using an external screen with Bela, which can be embedded in your project and powered directly from the Bela board, like the OLED screens above.
Then there is the GUI functionality which is more about creating visualisations or control interfaces which run in the browser, alongside the IDE. The GUI is launched in a way similar to the scope, it launches in a new tab of the browser and uses p5.js for the drawing and creating interface objects etc.
There's more info this here: https://learn.bela.io/the-ide/crafting-guis/
This can be used for waveform or parameter visualisation, sensor debugging, or for creating control interfaces without having to connect up hardware. However, it needs a computer with a browser in order to launch the GUI. It's possible to make this page pop up by default which can be very useful for creating a configuration panel for changing sample sets or switching between presets. I've used this in the past for handing an instrument over to musicians who could then do a bit of extra configuration by plugging the instrument into their laptop and opening the browser.
I feel that this kind of thing is not what you're looking for though. If you want an embedded option with a bigger screen then you could also use an Arduino or other microcontroller to drive the screen. The Arduino would be plugged into the USB host port on Bela and you can send it messages over Serial from your PD patch.
