How would he fair today, and would he be a heavyweight?

He would seriously struggle today just because he was naturally a cruiserweight ( if that now cruiser is 200lbs ). I feel the power and size difference would be too much for Rocky but he had the heart of a lion and could probably outwork a lot of the contenders today.

However in the 1950's, 6ft 5inch heavyweights were similar to the Valuev's of today. As decades have gone by, people have grown naturally larger so in todays standards Marciano would probably be 6ft 1 ish and about 15.5 to 16 stone. At that weight hed beat them all. Nobody could keep him off.

Sorry to totally disagree, but if Marciano was 6' 1" ish and 15.5 - 16 st, he simply wouldn't last 5 rounds at the pace he fought at. The fact that he was so light is what allowed him to throw so many punches, his iron chin notwithstanding.

if heart determination and stamina were the only ingredients to winning fights then marciano would stay unbeaten in any era.
you cant take away his unbeaten record but he only defended his title 6 times against either mediocre oppostion or fighters past their prime..even *ancient* archie moore was able to knock him down and archie was only a lightheavy.

He would have been too small and been a cruiserweight. He probably could have won titles but in todays time they stop fights way early. the first sign of blood, after a knockdown without giving a count, or even if an opponent throws a bunch of patty pat punches without a punch in return. Marciano would be a victim of the times and not been undefeated.

200lb weight class would've been ideal for rock

Marciano wouldn't survive in the heavyweight, he didn't even fight the best in his era...all old men, but don't me wrong he can have a chance in the cruiserweight...dream fight is marciano vs james toney

he would be a criserweight i think

he would have struggled a lot..
he is simply to small. he would be crushed by both the Klitschkos and Lewis mainly because of the size difference..

gotta say though, HUGE fan of the rock!

I've often read in recent years about Marcian being a 'small' guy, would only be a cruiserweight etc.. well, one thing to remember about Marciano is that his walk around weight wasn't 185lb, that was his peak-fighting weight, but alot heavier- something around 200-210lb's depending on things.
Rocky was as hard a trainer as you could physically be- his entire boxing style and method depended on his stamina, strength, punch resistence and sheer physicality to overwhelm and break the spirit of his opponents. Look at photo's of Rocky, there a classic photo with his hand being raised by the referee post-fight, and you can clearly see that his neck is extremely thick in comparison to his shoulders. Thats because few heavyweights, probably none in fact, ever 'trained down' for a fight like Marciano did. Marciano probably couldn't have lost much more weight without having weakened himself, his trainer Charlie Goldman said he'd never seen a trainer like Rocky.
Most heavyweights get in shape or fit to go 8, 10 or 12 rounds or train to get down to a good weight where they feel strong etc., Rocky would get down from being a very strong, big-boned Italian to 185lb's by running, punching and training his guts out. You dont think he could chase Walcott all over the ring and in the 13th round KO Walcott with a single right hand if he wasn't phenomanally fit, and that fitness corresponds to him being svelt...
People today have forgotten what a fit heavyweight looks like.