If there's any company that has tried to push the boundaries on how big a smartphone should be, that's definitely Samsung. They pushed the boundaries with some of their earlier 4.5" phones, including a version of Galaxy S 2 on T-mobile, then with the Galaxy Nexus, which was 4.7", and of course the 5.3" Galaxy Note, but that one I think can be considered more of a small tablet than a smartphone.
But as far as phones go, the Galaxy Nexus is certainly considered the biggest one, although it's not that much bigger than a 4.3" Galaxy S2, and the reason for that is that the Nexus lacks physical buttons, and the screen now basically occupies the space of the where the physical buttons would me, therefore making the screen larger than on normal phones. This is why a 4.7" phone is not that much larger than a 4.3" one.
Samsung is rumored to make the Galaxy S3 a 4.8" device, but probably doesn't mean that it's going to be even larger than the Galaxy Nexus, but instead it's going make the Galaxy S3 an even tighter device, with very small bezels, or none at all, so it's all just a slab of glass. It's also rumored to have a ceramic backing (I'd assume a very tough one), so that should make it a very interesting device, if done right. I could imagine how it would at least feel very good in hand.
Samsung has long been criticized for offering plasticky phones, and it's long overdue that they try some new material. HTC's aluminum phones feel great in hand and are solid, Motorola has tried kevlar on their new Razr phones, and of course Apple is being lauded for using glass in their phone, even though that makes it extremely prone to shattering, but at least it sounds good in the press. Having a plasticky phone does't sound as exciting.
Samsung is not going to unveil the Galaxy S3 at MWC, but I think we can expect more rumors to come out about it in these few days.