They wear special g-suits to somewhat constrict blood flow. When they perform G maneuvers they do have to flex and hold certain muscle groups, including their breath and take in small gasps of air - think constipated shitting - to keep blood from rushing out of their head and blacking out. So yes, for this reason, tall fighter pilots are rare and smaller pilots are typically better performers.
You wouldn't think it, but fighter pilots do have to stay in great shape and take an absolutely ridiculous toll on their body from what they do. I had a teacher, retired air force pilot, who looked about 15 years older than he looked pretty worn out. That constant G exertion on the body does serious damage to blood vessels.
Here's a short demonstration of the kind of training they do, I'm sure you can find more if you look around. Looks pretty awful.