The differences between a free safety and strong safety have begun to blur over the last year, with both players needing the ability to cover and stop the run. In years past, a strong safety could survive without great coverage skills, but spread offenses have made it a requirement that both safeties be strong in all aspects of the game.
It's important for a safety to be fast and fluid, but all the speed in the world cannot cover up for a safety that cannot quickly and correctly diagnose a play. How quickly they move toward the play—whether that's coverage or the run—and if they make the right move toward the ball. Being able to play the ball in the air and react on time.
Don't care about size or strength. If you don't know what coverage you should be in. Your going to be lost.
On any given play, a safety will be tasked with running the alley to stop the run or turning and running in coverage. More than any position on defense, safeties are constantly moving toward the ball, and they have more ground to cover than anyone else on the team to get there.
There are safeties without great speed, but those players make up for it with exceptional instincts. It's good to have one or the other, but the best safeties in the game have been a combination of instincts and athletic ability.
Generally speaking, what you see is what you get.Safeties do play in different schemes, but they're asked to do things—man and zone coverage. A free safety will take the same drop steps into his backpedal on zone coverage. Same for a strong safety locked up in man coverage. This is what helps make scouting safeties an easier proposition than quarterbacks or cornerbacks.
Size requirements for the safety position can vary depending on which team you ask, but the general rule of thumb is taller than 6'0" and heavier than 200 lbs. That can fluctuate, of course, but your dream safety who never leaves the field has to be big enough to stop the run and cover the field with range.
Scouting safeties often comes down to personal preference. Do you want the 4.3 burner with range and ball skills, or the 220-pound freight train knocking ball-carriers senseless? It's rare to find both in a prospect, and if you do, bet on him being a top-15 pick.