So you can control the fee rate but size depends on your UTXOs.
Virtual size or weight. Although, if you are referring to legacy address, it is size but the size is the same as the virtual size, so far it is legacy address.
Also, it is very important to remember that taproot transactions are less supported across wallets and explore. Which can raise concern or even rejection if you are using a more advanced setups like custom scripts or PSBT. So yeah, it is the latest upgrade but it cannot be all efficient to everyone because most individuals use Bitcoin differently, some prefer custom setups.
The major advantage of taproot that was proposed was multisig which will significantly reduced it transaction fee even far more than if compared to legacy and segwit multisig. But I am surprised BIP86 was introduced which is single signature of taproot and we have not seen it multisig.