What BTU heater for this size Garage?

Hi everyone! I'm getting ready for winter and I'm going to put heat in this thing before it gets cold. I've decided on the Northern "Big Maxx" heater. I figure it will be neat and clean and heat things up quickly.
My problem is the proper size for my situation. There are 45,000 and 75,000 BTU units.
My garage is 30x40, 12'6" high ceiling, 2x6 walls w/r19 and r30 in the ceiling, 4 windows and three large bay doors that are insulated.
Can anyone give me an idea/opinion on which of these sizes would be best? Will the 45,000 take care of my needs with the insulation or should I opt for the bigger unit? Winter here gets down to around 15 to 20 on occasion.
Here is the build
http://www.garagejournal.com/forum/showthread.php?t=51650&highlight=brassspike
Thanks! Bill

http://hearth.com/calc/btucalc.html

You could use that calculator. I put in 45x40 (since they assume 8' wall heights, I added 50% to another dimension). They claim 46000 btu would be fine in a warmish climate.

You have just under 15000 cubic feet to heat, with your good insulation the 75K should be fine for a 45 degree temp. rise.

Here's the sizing guide from Mr. Heater :
http://www.mrheater.com/upload/10-27-08 MH 2008 NT Heat Guide.pdf

I have a 26' x 30' x 9", about 10,000 cubic feet and 45K works well, your place is larger, definatly go with the 75K

Update: You are in a warmer area, if you want to go a bit smaller the Modine Hot Dawg and Reznor come in 60K links below.......
Modine:
http://www.djsonline.com/modineunitheaters.htm
Reznor:
http://www.djsonline.com/reznor_UDAP_Unit_Heaters.htm

I'm in Minnesota and I have a 45,000btu in my 24x34x11 and it does just fine. R19 walls R30 ceiling and about 50% of the wall area is common to the house and the garage doors are not the best. Do a heat loss calc.

Don't go too big on a heater, my HVAC contractor says he'd rather error on a size smaller then larger, that way the unit doesn't short cycle. You'll be surprised if your garage is well insulated with good, high quality insulated garage doors, how well a properly sized heater, heats the space. In my case for my Reznor, which I think is a 60k BTU for a 1050 sq foot garage with a heated bonus room over the garage, with insulated CHI garage doors, in the coldest of Colorado winters (last winter wind chill -23 F) I could keep the garage at 60 degrees and the unit would only cycle 3 times a hour at most.