Just pick an industry or two that you want in your final layout, and build a diorama around them. If you go to any home store, they'll usually cut plywood for you in whatever dimensions you want (up to a certain number of cuts) for free.
So grab a 4 x 4 of 1/2" or 3/4" plywood, and pick a size, and go for it. I might start out with something smaller but wide enough to do some damage - maybe 14" wide (could fit on a shelf then), and maybe a couple feet long.
If you know the era and location that you want to model, just start brainstorming different things you might see there.
So, for example, I'm modeling 1980 in Kansas/Oklahoma/Texas. For the 12" x 12" competition in May I was throwing around some ideas, and I've come up with two options: (1) a rural chemical distributor; and (2) a downtown scene in Amarillo TX, with a fast food taco restaurant, a Sonic, and a Tower Records, and maybe a coffee shop.