Slightly bigger than a 35mm film canister.
Please be VERY careful when pulling off the road, as well as getting back on the road, as bikes and cars exercise "WOT" all the time!
While placing the cache, we noticed that this stretch of road is long and straight and it is perfect for "WOT" driving.
A definition of WOT refers to "an internal combustion engine's maximum intake of air and fuel that occurs when the throttle plates inside the carburettor or throttle body are "wide open", providing the least resistance to the incoming air. In the case of an automobile, WOT is when the accelerator is depressed fully, sometimes referred to as "flooring it"."
Acknowledgements: http://en.wikipedia.org/wiki/Wide_open_throttle.