The MercuryII will receive c-band digital FTA. You'll need a c-band lnb/lnbf in order to receive c-band though. As for the dish size, you "might" get a very strong channel if there is no other interference, but it's doubtful that you would get anything reliable (if anything at all). 6ft is generally the minimum c-band dish, with 10-12 foot being desirable.
IF you go trying to get c-band with this dish (and I wouldn't waste my time, given that you can find larger dishes for free if you spend some time, and you'll be MUCH happier with a larger dish), some will steer you towards the bsc-621, or other similar lnbf setups. My preference is a good old dual c-band feedhorn with separate lnb's. I've had 5-6 of these cheap pot-metal lnbf combos and none of them have performed as well as a 20+ year old feedhorn with lnb combo.