Don't think in terms of "minimum" radius, but in "maximum", go as big as you possibly can. Don't cram everything known to model railroading on your layout, can't be done anyway.
Also, forget scenery for 1 year. Put track down and run trains, you're going to find things that need to be re-worked guaranteed. I've got 4 sidings on my 13x30 layout and they've all been relocated at least twice, that would have meant removing scenery if it had been in place. Doing it this way will also let you know of any mistakes you made in trackwork which will be easier to fix.
I'm on my 4th or 5th O-scale layout, 3-4 HO layouts prior to going large.
