An earlier thread discussing G-Shocks had mentioned the size. A few people get nervous about getting one as they have a reputation of being HUGE. There are a variety of sizes and I dont really find them over large.
Here are some comparison photos against a couple from the big I. An original 40mm Pro Diver and a 47mm Pro Diver Off Shore.
Sorry, the photos are not all that great...took them outside and its cold and windy so I hurried.
Gulfman is listed as 46mm
6900 series at 50mm
mtg-910 listed at 46mm
mudman at 46mm
rescue series listed at 50mm
5600 series...probably the smallest of the standard G line at 42mm