[Patrick]

If you want big wheels, some people used 12'' records.
If you can use more than one at a time (say 3) then you could use big wheels with a really long lever on each one and set them of at the same time. I'd be like gearing it up a lot, but you'll have 3 times the power to cope with the big wheels and gearing.
OR...if you wanted to make it really complicated you could set the traps up to work one at a time (when one finishes it triggers the next one). Wind the second and third strings onto a one way bearing that fits on the axle (so the string doesn't get unwound by the axle turning). You'd have to have something keeping the bearing from turning until the trap is triggered, so the string doesn't get unwound again, that would probably be the hardest part (as long as you can find the right one way bearing easily). Since getting off the line takes the most power, the second and third traps could use bigger levers (same as a bigger pinion), since it's already got momentum.
3 mouse traps would have the power to go way more than 100ft if done right.