But how does it apply to personally owned wallets with only 1 signature? Is there any data comparison if the transaction size after upgrade in this case could be bigger than before?
To know more about pay-to-taproot (P2TR) transaction virtual size, you can check this, it can be of help.
Pay-to-taproot (P2TR) transaction feeOne thing about P2TR is that all transactions (be it multisig, closing of lightning network, swapping and any other transaction that make use of taproot) can have the same fee as just as single payment wallet. So if taproot also support just a single key (but I don't know yet), the fee will still be the same. Only what can alter the fee are the inputs and outputs of the transaction.