If it is just an area the horse can use to get out of the weather: yes, 6'x12' is fine.
If it were to be used as a stall: no, the smallest space your horse would be comfortable with would be 10'x10'.
Yeah I not gona build an actual stall to keep him locked up in. I will though have an area about 20 x 12 around the 8 x 12 shed ( decided to go 8x12 with 12 ft side being the width). That 20x12 area will be the shed yard where I will keep him at night. He will be turned out to the pasture during the days.