How to calculate tire size?

Put the 14 x 7 on the back with 225/70R14, put the 14 x 6 on the front with 215/70R14. You can use a 195/7514 tire for a spare on the 5.5" wheel. But it won't quite fit in the well, depth-wise. You need to keep all the tires about the same diameter. These are all around 25"-26" dia.