Johnny Unitas was the best QB in his era, he was 6-1 195 pounds. Fran Tarkenton who retired as the all time leading passer was 6-0 185 pounds. Monster fullback Jim Taylor was a 215 pounder, the great John Henry Johnson (fullback) was considered the baddest back in the league was all of 216 pounds. The best receiver in the league in that era was Bobby Mitchell he was a 9.6 yards guy but that's about a 10.45 100m today. That won't win most big HS meets in Texas/Florida/Cali. He was 6-0 195 pounds.
The huge difference in todays game is that size with speed combo, big guys who can fly. Back in the day that was rare, today it is the norm.
Look at all that was said about Russell Wilsons size, he was a big as Unitas, Tarkenton, Bart Starr all in the Hall of Fame. Nobody talked about their size way back when.
There were no 6-4 receivers with 4.4 speed back then, even the college sprinters like Bob Boyd, Ollie Matson, Buddy Young who played in the league were nowhere close to the speedsters we see today, Jamaal Charles would blow by any of them and he's not close to being the fastest NFLer today.
Today linebackers would have been D linemen back then, safties would have been backers. A 6-3 QB would have been an alien.
Bob Hayes could run with these guys today and Gale Sayers could play, Jim Brown could play but after that....hmmmm?