Brother, 660cc cars give the same fuel average as you are already getting. Don't think that you will get anything more than that unless you find a manual car. Secondly, the spare parts are NOT cheap as you may think considering its a small car. In short, changing your coure for economical purpose is not viable. If you want space, do change the car but then, as some members have already suggested, go for a passo or Vitz. Same fuel average but more comfy and spacious.
As far as manual car is concerned, i would suggest you to drive an auto first and then decide. You can drive auto just like manual if you know how to drive. You can totally control when you want the car to change gears.
Hope it helps