are you one of those guys who logs onto 2p2, asks a question like this, then logs a half hour of "study" in your poker journal?
it's a bit weird to ask about exact frequencies in spots that can depend on many variables.
clearly it's a texture which is quite bad for Hero's range relative to Villain's, so in general we should do a lot of checking, and against better/more aggressive opponents, probably should be checking 90+%. against weaker regs and passive fish, you can probably get away with something more exploitative.
I don't think 66% is favored size on this texture, anyway. Our cbetting range is very nutted/polarized, so using a medium size really makes zero sense for that spot.