I’m 6-foot8 and have experimented with longer crank arms in the past – I think they were 190mm. One of my issues was my right foot was really close to and would occasionally hit the rear derailleur. What can I do?
That can be a problem, but it is remarkably rare. I find that most riders, even with size 14 feet on a 215mm crank, do not hit the rear derailleur, even with a short chainstay on their frame, if the bike fits them.
Tha tis because the ankle rotates in the vertical plane (the toe points down) as the foot pulls up on the backstroke, and it clears the derailleur. This is why triathletes with super-short chainstays and 26-inch wheels do not hit their heels, nor do most riders on short-chainstay time-trial bikes with seat-tube cutaways for the rear wheel to tuck into.
One problem can be that if your saddle is too low, your ankle will be flexed too much on the backstroke, and it will hit. This is often a problem with somebody your height, because most bikes you can find will not allow you to raise your seat enough to reach full leg extension, even with an extra-long seatpost.
Also, if you are using toeclips, your feet are likely too big to go as deeply into the pedal as they should be, so your heels stick out further back.I think that you will find that anyone with big feet, no matter the crank length, can hit the rear derailleur with their heel if their foot is flat.
Back in 1980, I totalled a wheel and rear derailleur by bunny-hopping a train track on a Masi Gran Criterium. My right foot was back and level as I jumped, and my heel pushed the derailleur into the spokes, stopping the wheel in midair. When I landed, metal parts flew everywhere, and my wheel was D-shaped from the spokes mangled by the derailleur pulling the rim in. That was with a 180mm crank, but I had never hit the derailleur when pedaling, and now I do not either, even though I ride a 205mm crank and have size 12 feet and 41cm chainstay length.
I have a lot of overlap with my rear derailleur, but I never hit it when pedaling. And it is not that my heel clears to the outside of the derailleur; my heel is pretty far in toward the crank (i.e., more duck-footed than pigeon-toed); it clears forward of the rear derailleur by rotating forward.
I am more careful jumping, though! I try to bunny-hop my bike with my right foot forward.
With a bike that fits you properly (which admittedly is a challenge for people your or my height – that’s why I started building custom frames and custom cranks!), if your foot still hits, then you need either a longer chainstay on the frame, or you need a wider pedaling stance. Moving yours inboard on the shoes would be the first thing to try. After that, a longer pedal spindle or longer bottom-bracket spindle may be required.