I've found that radius is also closely related to the type of layout you are building (real duh, huh). It seems that a minimum radius is more critical for a peninsula or table layout where you are trying to squeeze track within a width constraint. While designing my layout, which is an around the wall shelf type, radius doesn't play that much of a role. It does, but it's not as critical. My room is 7 x16 ft, which means that I can get away with the smallest radius possible up to a 40" radius, if I want one big sweeping curve. Even if I try to use the smaller radii, I never seem to net any major gains. I can gain some realestate, but not as much as I would have thought.
My layout is also relatively high with the mean track height at 55", or just below eye level for me. It's only 18" deep, so reaching something at the backdrop is not bad. Having the layout this high has the advantage of straightening out a radius, so what may be a 24 inch radius, may well be undicernable from a 36" radius. I can't see using super wide radii, just because I can, neither. So, in my case, I'm going to shoot for no less than 30"R for the main for operational purposes and whatever for the branchline.