I'm 5'5" and about 300 lbs. I recently had the opportunity to fly a 737-800 simulator. I was initially worried about getting into what looked like a quite snug seat.

The 737 pilot and copilot seats are mounted on J-rail platforms. They are normally moved full outboard and full aft to allow the pilots to get in and out of the seats, and then adjusted forward and inboard, to allow them to reach the rudder pedals and be centered in front of the yoke.

As it turned out, the cockpit was actually quite comfortable for me, and the 737-800 is a VERY enjoyable airplane to fly. Demanding, yes, but rewarding as well.

Edited in March 2017, primarily to correct the point about seat mounting: Across the Christmas/New Years week, I put in 8 solid hours in this simulator. Between the J-rail seat and the handhold above the cockpit window, getting in and out of the seat was no problem at all. My one big problem is that flying the 737-800, even in simulation, is HIGHLY addictive!