Hi
I think we have to stick with the rough idea that is already accepted, otherwise we may alienate existing enthusiasts.
By going for 500cc you would be excluding Fiat 600 etc. that would normaly come along with the Fiat 500's There are probably other examples that would no longer qualify.
My personal opinion is roughly 750cc, but with the emphasis on vehicle body size being more important, for example, Jean has a car in the barn, can't remember the type (Is it a Goliath ?) that she terms a microcar, but it has a massive 4 door saloon body, so I don't think it is the normaly accepted concept of a microcar.
Anyone out there got a big stick, so that we can stir up this can of worms?
All the best
John