Which Raspberry Pi and memory size and what size SD card do I need?

I have Home Assistant running on my Raspberry Pi 4B…upgraded from a 3B. Honestly, the upgrade was because there was a 4B, not because I had anything suggesting I needed more processing power, etc. That got me thinking…how does one know if they are pressing the bounds of their Raspberry…eg. which memory model do I need? what size SD card should I be using? How much memory do they really need on that pi?

How does one figure out when to upgrade (of if they could downgrade) their hardware of SD/SDD storage?

Note: I’m not looking for recommendations…I’m looking for the tools/procedures that could give me information about my installation.

You can install something like Glances to look at how much CPU, RAM and storage is being used.