Just want to heat to keep above freezing for a few hours once in awhile to dethaw snow equipment.
Can get negative 30-40 at times, Northeast Iowa

With those two statements I would not worry about the -30F and colder since it is so few hours every year it is not worth worrying about when you are warming the shed once and a while for a few hours at a time to thaw snow equipment (dethawing would be freezing them up.)

Using realistic numbers helps.  According to the 2021 ASHRAE tables for Decorah, IA is in the NE corner of the state, the 99% heading DB design temperature is -4F.0F which means 99% of the time the temperature in Decorah IA is above -4.0F, given there are 8760 hours in a year, 1% is 87.60 hours per year the temp is below -4.0F which would require a temperature rise of 36deg F to keep the space above freezing; then look at the 99.6% temp of -8.5F which means 0.4% of the time or approx 35 hrs per year the temp is below -8.5F which will require just over 40deg F temp rise to keep the garage above freezing.  You can then look at the Return Year Periods for extremes and find that -20.0F is 5yr, -24.2F is 10yr, -27.7F is 20yr, and -32.4F is 50yr for Decorah, IA.  I don't see -40F happening very often there.

Now say you want to raise temp from -0F to 40F that is a 40deg temp rise and 24,000 BTU (approx 700W) heater should have no issue warming a single car garage by 70F since you want just over half that a 4000W heater should keep the garge above 40F when temps are at or above 0F.  The trouble is you will need to warm the vehicle being thawed and it takes energy to melt ice so you may very well want 2-5x that size just to speed the process.