I believe the i-MiEV front wheels are 4" wide, and the rear are 6" wide. 15" wheels all the way around, but different widths. I don't think a 14" wheel will clear the front brakes, but should work on the back.
I think Joe found that the spare tire for his former Honda Insight fit the front without modification. Other members have gotten different wheel/tire combos on the car, but it required spacers and changing out the lugs.
There is a OD ratio that is very strict on the i-MiEV. A replacement tire/wheel combo must be within 3% of the original tire, otherwise regen braking is severely crippled. I put 175/55R15 on the back instead of the stock 175/60R15, and this was too much of a change in OD. Some possible sizes that should theoretically work:
Front:
155/60R15
165/55R15
Rear:
185/55R15
165/65R15
Mitsubishi had to get creative in making the i-MiEV understeer. When they were developing the car, having the same size tire for front and back caused the car to oversteer.