Maybe measure from the front edge of the front tire, to the contact patch of the rear tire. The rest of the bike can hang over the tailgate.
Will the bed extender support the bike?
You're going to need a lot of help getting a bike up that ramp. Even with a wide, smooth ramp it can be tough. One man won't do it, and you won't ride up that.
Maybe a 2' wide piece of plywood down the middle of the HF ramp, would make it easier to push. Would be nice to have 2-3 people though. It's 5' 8" long, so will be kinda steep on the incline.
If I had to try it, I would try to find a slope so the ramp would be flatter. With enough help a nice long board would be ok for a ramp. What's the longest stick of lumber you can haul?
Once pushed a wrecked Goldwing up a 2x6 board, onto a lifted 4x4 truck, but had some help. All I did was steer, had road rash bad. arrgh
Use good tie downs, the cheap ones break at the worst times.