I have three heaters for my Clam Voyager. (2 Man)
The first heater is a sport cat, that I use when it is very nice out and just need to warm my fingers.
I then have the regular buddy heater, which I use 85% of the time. It maxes out at 9,000 BTUs of heat.
I also have in my truck, for really cold days the Big Buddy heater. It maxes out at 18,000 BTUs of heat.
In a one man shack, the regular buddy heater (which takes one small tank) should have you covered on all but the most bitter cold days.
The Big Buddy takes two small tanks and a lot more space.
Hope this helps,
Wiener