I would concur with the comments of at LEAST O-72 curves, especially with as much room as you have.
My last layout was 14' x 17', a O-63/O-54 (gargraves/ross) double-main with lots of sidings (22 RCS turnouts) and a 29" turntable. It was lots of fun. Great layout to run my scale Pacifics and Mikados. Not so great, however, for my scale T-3 (due to an S-curve) and I could NOT run any of my scale passenger cars. I was in the process of building a 22' x 17' extension to the layout and replacing all the O-63 and O-54 curves when we decided to move. My current layout in our rental house is a carpet layout in my 12' x 17' trainroom. This time, I went with the largest Lionel Fastrack curves that I could buy, O-96 and O-84. Now I can run ALL of my trains!
Jim