I've loaded just about everything you can load on the smallest cross-bars that Yakima makes. I got the shortest cross-bars because I did not want them to stick out on the sides. If you need to put something larger up there, get ones that stick out a bit on the sides. Just take them off before you go off road or you'll be snaggin' everything.
I've had one tandem 12.5' kayak loaded up there, water side down, and gear bags secured in the cockpit.
I've had a 12.5' tandem and a 13' tandem up there, water side against my Yakima Kayak Stackers (strongly recommended if you have sit-on-top, or "ocean" kayaks). I do not recommend these if you have a sit-in kayak (they also don't work for canoes).
I've maxed out the short Yakima crossbars and Kayak Stacker by putting 4 kayaks up there (shortest about 9' and longest about 11').
Here are the Yakima Kayak Stackers, with no kayaks on them...
I've also had my 13' white-water canoe up there. It does hang off the back quite a bit. Center it over your cross rails, not the bumpers. It will stick out the back a lot, and will barely reach the front bumper (if at all). The main tie-downs should be under the rails and over the body of the canoe.
Don't use the bow and stern lines to hold the canoe to the truck. If you do this, you will bow your canoe. And, while a good amount of rocker is suggested in a white-water canoe, this is not the way to get it. The bow and stern lines are for emergencies only. They keep the canoe from launching and killing someone, should the main straps fail.
Measure the Gunwhales (top edge of the sides) at the midship thwart (wooden bar that goes across the center of the canoe), at the stern thwart, and bow thward. You will most likely be tying down around the stern and bow thwarts, so this measurement is of incredible importance. Get a set of Yakima LowPro cross rails that are wide enough to handle this measurement (with a bit of excess on either end) and you'll get an amazing set of tie-downs (at least, my Yakima LowPros came with quality tie-downs).
Here's a little chart to help anyone who doesn't know canoe parts...
I'll post some picts of my 13' white-water canoe on the KJ tonight. I lose a decent amount of foward vision because my canoe has a lot of rocker. But most every other canoe has less rocker than mine, and less bow and stern lift, so you will most likely have more visibility than me. And I do fine.