The Galaxy S III is 2.8 x 5.4 x 0.34 in., making it slightly longer and thinner than the HTC One X or Samsung Galaxy Nexus (U.S. versions). The phone weighs 4.7 oz. -- a hair more than the One X and 0.4 oz. less than the U.S. Galaxy Nexus.
The new Galaxy doesn't feel overly large in the hand, which is quite a feat when you consider its supersized 4.8-in. screen. Samsung has managed to include a large display while still achieving a sleek-feeling form; the phone is well-designed and quite comfortable to hold. The glossy back feels somewhat slick compared to the textured material on a phone like the Galaxy Nexus, but despite my initial concerns, I never felt the device slipping from my hands.
One thing I did feel was heat: Both the white AT&T model and the blue T-Mobile model occasionally became quite warm to the touch during use (and not even resource-intensive use -- just casual Web and social media browsing). The phones never got so hot that I couldn't physically hold them, but they got hot enough that I was acutely aware of the temperature, both on the back casings and on the glass displays.
Speaking of displays, the Galaxy S III packs a 1280 x 720 HD Super AMOLED screen. The screen looks good: Colors are bright and blacks are satisfyingly deep. Display aficionados may balk at the Pentile-based nature of the technology, which is generally considered to be less impressive than the LCD-based alternative seen on phones like the One X. While there's certainly merit to that argument, it's hard to call the Galaxy S III's display a weak point; even looking at the Galaxy S III and the One X side by side, the difference in display quality is difficult to detect. Unless you're finely tuned into subtle display differences, you're going to be pleased.
A warning, though: The autobrightness feature on the phone isn't so great. The display frequently adjusted itself to a too-dim setting during my use, which proved to be mildly irritating. (You can, of course, set your brightness manually, but that sort of defeats the purpose of having an autobrightness tool.) Hopefully Samsung can correct this via a future firmware update.