How do I know what "storage key" value to provide?
evm.storage is a good tool for this. Just type in a contract address and the storage layout will be shown. The slot of a specific state variable looks like 0x0000...0002 in the "Storage" tab. The storage key of that slot is simply keccak256(slot) for a single storage variable. But it can get much more complicated than that if the variable doesn't take up 32 bytes and is grouped with another one, or if it's an array or mapping. You can read more about storage layout in general on the solidity doc website.
Troubleshooting Common Errors
"invalid witness size" / "witness length is invalid"
If you are seeing witness size related errors, try cleaning your circuit output directory and recompile/setup your circuit.
