Phil,
Make sure you pick a location and route for your cable to allow for the easy removal of your battery.
In the picture above, I have to lift the battery up, move it towards the center, and then lift it out of the plane.
With the oil pressure line running where it is, I figure it best just to parallel it with the ground strap.
(I can't give you a length because I have a longer than standard engine mount for light engine.)
One other thing, we like to see a secondary ground path from the engine to the fuselage. In my case, the oil and fuel pressure lines serve this function. I don't want to start that debate over again (it is on another thread, someplace), just want to give you an idea of a good practice. The secondary wire can be fairly small, just big enough to keep things running in flight, should your ground strap fail.