"Vicinity" ISO16593 RFID can achieve several meters range under circumstances, but only using large reader coils and several watts power. UHF RFID can also achieve larger ranges.
In both cases, the requirements for the tag antenna will probably not fit the "small key" idea. But you should at least study the recent technology to know why your project can't be realized.
An active tag (a battery powered transceiver) can better achieve long range and small size.