I put 8' and 10' inside and out window all the time. Anything longer goes up top. Would suggest not using the rear glass as a holding mechanism though. Don't ask.
Depends on if your going for completely inside or if your willing to let it stick out the back. 8 feet will fit inside with all the windows up but I have personally carried 5 12 foot 2x6s in mine with the rear window down and it worked fine.