Kenmac:
Your part way there, you can go wider (I think again) but not taller or shorter. Taller and shorter tires will change the rolling circumference.
Think of it this way, while your setting there and reading this get a quarter and dime out of your pocket and roll them one revolution across your computer desk bet you the quarter will cover more distance than the dime...well did you try it...was I right? Now think of the quarter is your back tire and the dime is your front tire.
The new tires you replace the existing tires with will have to roll the same distance as they do now. Now with that said....I bet your setting there and thinking "can I incress both front and back the same % in relation to what they are now" and to tell you the truth I don't have a clue...it's make my brain hurt to think about it!
The rolling circumference was set up by some guy with way to much time on his hands back in Japan at the Yanmar factory, and he's going to be real pissed if you change all his math and calcuation on his design, and just might show up at your door with a calculator in his hand and a pen protector in his pocket and screaming at you!