You will probably want to upgrade from a rpi to a pc shortly after trying things out with the rpi. So it doesn't really matter if it's a 2TB or a 4TB external drive. When you upgrade from a rpi to some old pc you need to get some internal drives anyway. Could just do it straight away. Umbrel on a pc is easier to manage anyway imo. That being said bigger is better. Bigger means less hassle in the future.
Discussion
You'd think getting a fast SSD means fast reads/writes, but that's not the case if you have a usb bottleneck in there, like you do when running a node on a rpi with an external SSD drive. Internal drives are insanely faster and you really feel the difference in the initial block download. It's hours vs. weeks.
That was one aspect I was worried about in this setup; the capabilities of the RPi itself limiting what it can do with an external drive because of that bottleneck.
I have an old PC that would probably be more efficient, trade off with that is the required power use and more of a hassle to get going now - but probably less of a headache later and more “future proof” now with that difference in speed.
Decisions, decisions.