Looking at BMR and PennLUG etc. it seems most are building at 1:48 for a variety of reasons. This generally works out to 8 wide rolling stock with 8 wide NA diesels and 8 wide steam loco bodies (minus rods, handrails, cabs, etc.). 1:48 works decently for minifig height (not width), there's a reduction in parts/cost from 9 or 10 wide or 1:38/1:40, and it's easier to make 1:48/8W function on R40 layouts and switches. 1:48 doesn't look terribly out of scale compared to the tracks either. I personally picked 1:48/~8W as it allows finer detail than 6W, doesn't cripple the wallet, there's a somewhat accepted standard (so instructions/kits/examples are more available), and finally I can "kitbash" O or S scale decals and make them work for my models. I'm working on prototypical ~1950's L&N locos and stock, so AFOL vendors (BMR, etc) haven't gotten around to printing decals for my tastes yet!