what determins the size of car wheels and tyres?

Answer 1

The size of wheels and tyres are decided very early in a car's development programme - before the styling is completed, or the equipment levels decided.
They will be part of the 3-dimensional engineering "package" of the car, and will be decided taking account of:
- the car's proposed external dimensions
- the proposed engine and transmissions packages
- the proposed fully-laden mass (and mass distribution)
- the proposed vehicle performance targets (speed, acceleration, fuel consumption, off-road ability, etc.)
- the proposed suspension package
- the proposed crash structure
- targets for ride, handling and turning circle
- cost of parts
- cost of ownership (replacement costs)
- tyre availability
- marketing requirements (e.g wide, low-profile tyres on a sporting variant)

Once these have been determined, the "envelope" (i.e. the space required taking steering and suspension movement into account) for any proposed wheel and tyre size can be superimposed onto the 3D vehicle package, allowing the maximum wheel/tyre size without clashes with other components to be determined, and possibly some refinement of the package to accommodate larger wheels/tyres if required.

The wheel and tyre sizes will often be decided months before the gear ratios are determined.

Answer 2

The rims determine the size of your tyres, and the size of your rims is determined by the model of your car. the manufacturer should have a specifications on that, it'll tell you the recommended size and the maximum size you can get.
rims are freaking expensive!

Answer 3

Your manufacturer.

The designer figured in the size of the tire when they gears the transmission. So going too big or small effects your (a) speedo (b) acceleration (c) power (d) MPG (e) clearance