He'd probably overextend, go off balance and almost fall over, too. His body type is not really good for being a fighter. Of course, if he lands, you're going to be fucked up. If he misses, he'll likely do as I said and he'd also be spectacularly slow.
I'm not saying it's a fact or anything, just that people that large often go off balance when throwing hard punches and missing. And I'm not saying he wouldn't knock you out if he landed, he would easily KO you if he landed.
The truth is, if he was a fighter in any way, he'd get mauled. People that large often don't have great coordination as a professional fighter. That's why you don't really ever see people that large in combat sports and the ones you do see, they may be large but their body type is very different than what Andre's was.
He'd most likely end up like Giant Silva.