OK, it would appear the the above mentioned spare just might fit in a roadster trunk. ( not sure, haven't measured )
My question is what do you do with the flat 18/19" wheel and tire when and if you have to use the spare, and you have a passenger with you ?
Never mind, I have the answer - hide the flat somewhere, drive somewhere that can fix it, leave the passenger, go back and get the flat tire ( I you hid it well enough )
OR
put the top up, tie the flat on the top with some padding, turn on the emergency flashers and drive slowly and carefully to where you can get it fixed.