I would start by looking at all the background heat you expect to be generating in there- two humans, a fridge, some cooking.
Then do a basic heat loss calculation based on notional insulation values and the surface area of the hut. This will be dependent on outside temperature- do you want to use this all year round, or summer only?
Just to give you a data point, my little place needs no heating for about nine months of the year. The fridge, hot water tank, and a bit of solar gain seem to be enough. I've got u values of between 0.1 and 0.15, with the windows at about 1.0.