What should a big guy look for when buying a road bike? I'm 6ft 4" (1.8m) and 22st (139kg).  I do a fair bit of cycling, mainly spinning and mountain biking.

I would like to get into road riding so I'm looking to buy a road bike. I want to spend between £500-£750. 

I've had some issues in the past with pedal shafts rounding off the crank, especially when riding hard standing up on my mountain bike.

What things should I look for when choosing a road bike? Obviously a big frame, but what are the sizes? Also, a strong crank; are there any makes that are stronger than most?
Here's what I'd recommend

As others have suggested, you should avoid a stock bicycle. They're designed with the top of the bell curve in mind, and you're simply half-again as much man as the normal one. For more information on any of these topics, consult a bike shop!


Crankset: Since you asked specifically about them, I'll start here. The stiffest, strongest crank you can get is an a two-piece crank with an external bottom bracket. There are different types, made by different manufacturers, but they've been around long enough that they should all be durable (and as a bonus, better bearings!).
Sizing: The right size bicycle for you is determined by your leg length. A good shop should be able to measure you for the proper frame size. Along with stand over height, you should be measured for reach (which is a combination of your torso length and arm length), and handlebar width (measurement across shoulders, I believe).
Frame: Material doesn't particularly matter here. In the past, carbon has had durability issues, but that's largely been solved (so long as you select a quality manufacturer). However, with your budget in mind, steel or aluminum is a better choice.
Wheels: This is what you should be focusing a huge portion of your budget on. Quality, hand-spun, 3-cross wheels will last much, much longer than any pre-built wheelset.
Tires (tyres!): Be sure to select a wider tire than the old standby 700x23. I would even consider a 28, honestly. It's a small sacrifice in acceleration, but in terms of long-term comfort and cruising speed, a wider tire will be better.
Other components: What fits within the remainder of your budget. Road bike parts are super pricey, especially those fancy integrated shift/brake levers. Consider bar-end or even downtube shifters if you can handle it.