Hmmm... if you have a loco that needs a 6-ft turntable, then I doubt you can get it to go around a curve that is limited by an 11 ft diameter. This would limit you to a Point-to-Point type layout and 18-ft is a mighty short one... especially if you loco needs a 6-ft diameter turntable. Smaller locos would probably fit that size layout, and then you could do with a much smaller turntable and get more "layout" in the area.
I always check in on the 'indoor layouts' topics but this one got me really intrigued. A six foot turntable? Wow! I have a 35x40 foot indoor layout and I don't have room for a 6 foot turntable - or an engine that needs to ride on it.
Is that a typo? I expect that the 6 foot includes the turntable, roundhouse and a fan of tracks.
Still, it would have to go in the middle of the room so you can gain easy access from all sides. Track would have to go around the outside walls on a narrow shelf making the engine facility in the center the real show piece of the layout. An authentic, highly detailed display like that could work.
My previous layout was the same size as the current indoor one which is 10' x 19.5', its about the same area, on that I had a 1' turntable but no roundhouse only a facade of one. Engine facilites eat up a lot of space.
More description of the area in question and a clearer design intent of the layout is needed please.
What I would suggest is that you measure the longest Engine you plan on running, add a few inches and that is how large I would make the turntable that size, for example if the longest engine you would run is 20" I would build mine 24"~26", with only the feeder track lengths that are needed to get into the roundhouse.
Hmmm...if you have the whole room and don't mind a headknocker (duckunder), you might be able to wedge in a double loop that crosses over itself at one point, using either a crossover track or a bridge of some sort, and *maybe* wedge the turntable onto some sort of peninsula...but it seems like you're trying to wedge to much into too small a space.