Hmm. Front and rear plates?
Possibly, or maybe just one somewhere on the top tube/down tube.
What size -
Fairly small, smaller than car ones obviously - 4" by 1" maybe.
they would need to be readable by ANPR equipment, especially if linked to the insurance database,
Only ones that could be upgraded to use RFID, (or maybe ones that could theoretically use RFID, following a similar principle as TV detector vans).
and by normally sighted humans from 70 yards (or whatever is required in the driving test).
Would they be reflective, like car plates? Different colours front and back?
No, that probably wouldn't be necessary to achieve the benefit I'm proposing, which is largely a political benefit remember.
Could you transfer plates between bikes or have to have a separate set for each bike.
Separate set for each bike, like you have to for each car.
Would each bike then need to have a registered keeper - that way you could always claim it was someone else riding when the PC knocks on your door to ask about that red light you jumped.
Again, same for cars, and same rules for keeper liability would apply.
What about kids' bikes - at what age do they need plates?. Where would you mount them?
Whenever they want to ride on the road.