tire size big or small
 

If I put smaller tires on my car is that better or worse for mpg and if I have smaller tires in the front than in the rear or vice versa could this damage my car.

Well if your current tires are in decent shape I personally would just pump them up to whatever PSI your comfortable with. Otherwise there'll be more tires in the landfill.

If due for replacement, keeping them the same size would be best for alignment/steering geometry/tire rotation purposes. You'll have more caster if the front tires are smaller.

It shouldn't cause any immediate damage though, unless the bumper is dragging ;)

Depends on where your driving, all city/ stop and go smaller tires might help mpg. .
but if you do a lot of hwy then smaller might cause a slight loss.

Typically larger tires will have a higher rolling resistance, but also might give the advantage of lower RPM at cruising speeds. On the TDI forum larger tires almost always decrease the MPG.

My bad. You will indeed have less caster. Less caster can mean harder to drive in a straight line.

Peakster! how is straight line driving with the doughnuts on front?

It was a bit interesting ;). There did seem to be a bit more side movement at highway speeds, but I just thought it was because of the wind, and less grip on the ground.

Wikipedia: "A caster (or castor) is a type of wheel mounted with an offset steering pivot such that the wheel will automatically swivel to align itself to the direction from which it is pushed. They are commonly found on shopping carts and rolling chairs."