Normal rim will only fit profiles of 14' lets say 175/65 R14, 195/60 R14 etc.
Larger size which I recommended you will give two benefits more size means greater rubber which in return means greater patch and grip on road and slightly more rotational speed.
Buy some nice branded 15' alloys from kabar like shershah and bilal gunj will cost you max 8-10K and will greatly reduce the sprung weight of all 4 corners which will increase your fuel economy and reduce the engine load.
Keep you OEM steel caps in storage so you can replace them if u sell car.
Fuel economy depends on rolling resistance and pattern of tyre i.e. Michelin XM2 with 195/55 R15 would have much greater fuel economy and tread life than a Bridgestone Potenza GIII of same size.
If u have seen a RS version vitz of your model and shape you would notice a 195/50 R16 wheel :D.
Low profile large wheel size good for grip and traction bad for economy and tire life.
Small wheel size and high profile good for load bearing and ride quality bad for ground clearance and speed.
You have to balance in between.