On AT&T, you have a couple 4-inch options: the Atrix 4G and the Captivate Glide. Both of those feature the Tegra 2 processor, but it's the slower version of the Tegra 2 (still a good processor though). The Atrix is actually slightly smaller than the Nexus S, and the Glide is essentially the exact same size.
That said, I think you should consider the Galaxy S II. AT&T's Galaxy S II is the only US version that kept the originaly 4.3-inch spec, and it's actually very close to the overall size of the Nexus S, only about 1mm taller and 3mm wider.
The Atrix 2 is about the same size as the Galaxy S II but has more modern specs compared to the first Atrix, so it's one to consider, too.
There is also the HTC Vivid, which is only 4mm taller and 3mm wider than the Nexus S, but will get you a nice dual-core chipset with LTE connectivity.