27 inch or 700c

as long as your brakes are adjustable so they can reach down to the rims of 700c wheels it'll be fine, although i'd probably recommend changing both wheels to 700c..

I ran a 27 1 1/4 wheel on the rear, and a 700c on the front of a frame designed for 700'c wheels.

It was an old raleigh 531 road frame. It sharpened the steering, raised the BB slightly, had lots of benefits and felt good to ride.