This has come up on here in the past, IIRC.
SouthWest seem to have the right policy: Large pax are asked to purchase another seat at checkin, if they didn't already. Should there be empty seats on the flight, the price of the second ticket's refunded. Seems fair to all.
On a couple of recent flights, FAs have asked that people don't recline until everyone's finished guzzling - good idea.