I use 700x20's on my tri bike. Faster, but I notice one needs to be more cognazant of handling. Example=Rocks don't usually 'ping' out, bike tire actually kicks up & rolls sideways a slight distance. Also - I run them at 125+psi - they skid EASY! gotta watch corners and I touch brakes EASY.
Other road bikes I use 23's, then 28's on my cross bike.
I don't mix sizes.