Hi and welcome,
Within reason and depending on other metallic objects within the vicinity, a metal detector can by pretty large. The actual detector part of the equipment is generally a simple wire loop. Often multiple loops of a single wire, so you could make a 24" x 2" wooden former and wind your detector coil and then encapsulate it in epoxy etc.
Do some research on metal detector kits and I am sure you will find what you are looking for. There is quite a bit involved if the parts to be detected are moving as opposed to static. Also the inductance of the loop will need to be tuned for and with the circuitry involved. Another approach might be an array of Hall-effect sensors.
I cannot imagine how you would use RFID for detecting small pieces of metal.