Size hasn't really changed over the past two or so decades. Barry and Emmit Smith are a little shorter, but weigh the same, and have tremendously solid body types. Thermal, at 210 lbs would be the floor for a carry/touch machine. Going back further, OJ, Jim Brown, Marcus Allen, Eric Dickerson have elite size for today's game. Maybe there are more 6' 230 lbs guys in today's game, but they can become useless very quickly. If anything, they might be, on average, an inch or three taller than yesteryear.
BUT, skill has evolved a lot. Today, they are faster, more agile, and have considerably better hands and pass protection skills. They train year around, and need to compete against way better players on the other side of the ball. So throwing the all time greats in their primes like Simpson, Brown, and Dickerson on the field would be a tough one. Perhaps they would wear down quickly during today's fast paced, size and speed game.