How to calculate tire size?

I don't really understand what you are asking.

Look at the section width of the tires you are considering. They should be at least .25" greater than the actual width of the wheel. The more sidewall width you run beyond the width of the wheel and worse it will handle.

As far as height, above 26" in the front can start to get pretty tight, especially depending on the ride height.

As far as the spare, you can mount a tire at or slightly narrower than the overall wheel width and make sure to check the diameter of your spare tire well. I don't carry a spare anymore. CAn't fit rear spares anymore and AAA is much more convenient.

Last, I beg you to reconsider running 14's.