My best suggestion is to download XTrkCAD from http://www.xtrkcad.org/Wikka/HomePage, along with Getting Started tutorial. Enter the room dimensions and the location of the door and then try some different shaped layouts to see what fits best. My preference would be for a narrow (2' wide) shelf around the room, ending in two 4'x4' squares, if you have the room. This gives you the longest mainline running and most opportunities for switching and operations.
HO Scale Modeler