OK, on a vanilla road bike I am running 700x28 Schwalbe Marathon "Kevlar Guard" tires. They fit fine although need to be deflated to get past brakes--no problem.
Since avoiding flats is #1 priority, I ordered a set of Marathon Plus "Smart Guard" also in 700x28. Oops--they fit ok width-wise, but the built-up belt causes the tire to rub against the back of the center post and the arch of the fork and seat stay bridge.
To save $$ I order tires from Europe, so the "try and see" sizing guide doesn't work.
Does anybody know of a site that shows actual inflated dimensions of bike tires?
I'd like to try Conti gator skins and/or Specialized Armadillos, but who knows if they'd fit?