On the engineering side the lighter slats will help. Going from oak to pine will make them lighter but planing them to a lesser thickness is a better option. Going from a 6" wheel to an 8" wheel will not be that much difference other than lengthening the belt to match the extra surface area of the larger wheel. The right length belt matched to the right diameter wheel is really the same thing.

This is some Randy C. Stage III info he gave me a number of years ago. Take a permanent marker and mark the slat as soon as it touches the end roller. Make a mark on the wheel. Slowly roll the belt and mark the wheel just when the slat comes away from the wheel. If the belt comes off the wheel more than half way, wraps, the belt is too short. Add a slat or so. The slat should start to make the turn on the wheel when it comes off. If it drops off the end of the wheel and does not start to make the turn it will have to make a hard 90 degree turn when it goes back over the other wheel, which is drag. Remove a slat. The smaller the slat the easier it is to make this fine tuning adjustment. 2.5" would be a big increase or decrease depending on the length needed.

Or like Randy said, do what everyone else does. Find someone else's mill that is working, measure it from axle to axle, count their slats and allow for the slat spacing and all the math is done for you.

Best of luck. I enjoy the build as much as seeing the end product working a dog. Good luck. EWO