Articulateds (like Big Boys) always cause trouble. I agree with Matt's guideline of 4" from the center rail for wide-radius curves. If you're operating large equipment (like a 773) on small-radius curves, you might need to go 5" from the center rail. In other words, at least 40" wide platform if you are using postwar Lionel "O31" (which actually has a CENTER RAIL diameter of about 30"!)
Another important question: Is the benchwork free-standing in the middle of the room, or against the wall? If you don't mind the corners of your $1500 loco hanging out into the aisles, you can set the ties right to the edge of the platform. But if the layout is going against a wall, you need to be sure of the clearances before you lay your track. Measure with your largest equipment!