As small size 6x8 in UTH is fine for that size.
While low to mid 70s is recommended as a room temp many of us in northern climes can't keep our whole houses that warm all the time. My house hangs around 70 on the coldest days and I have seen my bedroom where I keep my snakes dip to 67 at night. As long as your warm hide is 85 your snake will find its comfort zone (ref: from the SMR care sheet on cornsnakes.net).
I personally won't let the room temp go below 65. If it goes below that on a regular basis, you might get a second UTH and control it with thermostat or rheostat to be around 75.
We had a little furnace problem on the coldest day of the year so far when I couldn't heat my house above 60. It was 6 hours before the heating guy arrived and fixed it. I placed 40 hour shipping heat packs, wrapped loosely in newspaper, under the cool sides of the vivs. It brought them up to 70-75 on the surface. Luckily the warm side UTHs had no problem holding temp even down to 60 in the house. Believe me, I monitored my snakes and viv temps carefully. I keep these shipping packs on hand just for emergencies like this.
Another tactic is to get a small supplemental electric heater just for your snake room to heat it up a few extra degrees. I recommend one with a thermostat and other safety features like tip over protection. It pays to pay a little more for a safer heater. You could also use ceramic heat emitters, but since heat rises it can be difficult to control the heat in the viv. You can also get radiant heat panels, which are quite pricey; but if you just need a few extra degrees a safety featured supplemental room heater might actually be a better choice to my mind. All heating devices have one or another safety issue either as a fire or burn hazard, so research and choose what works best for you. I do recommend a working smoke detector for any room in which any heating device is used.