There are a couple nice tire size calculators on the tire shop websites. I had to figure this out for a car a while back and just started playing with the different numbers until I found what I liked.
For instance: I'd enter 195/65r/15 and it would provide the precise height, tread width, sidewall, and suggested rim width. Then, if you want something a hair taller and wider re enter 205/65r/15. If the numbers were not quite looking right, I might try changing the 65r to 60r or 70r and start over with 205 then 215. The last number of 15 is just the rim diameter. If you drop it to 14, everything else changes so it helps to decide this first.