I have one of the 28k btu blue flame heaters in my garage. About the same size and inslated (minus the ceiling, which is sheet rocked). Keeps it nice and toasty.
I have it plumbed into the house (propane).
Im actually looking to sell it to buy a newer one with a T-stat. Even on low, it will get it warmer than the house in there, if the temps are like 30deg or above. Im hoping one of the t-stat ones will allow me to keep it on for a week or two at a time, without it being too crazy hot.
Dan