Electric is an expensive way of heating any space and a large double garage with tin roof and double width metal door is going to be lousy at keeping the heat in. I'd put a ceiling in about four inches from the roof and insulate it, if you can manage to do something similar to your door you'll have a better chance of getting it warm and keeping it comfy.
Personally I'd opt for gas, either a radiant heater like this
http://www.sealey.co.uk/PLPageBuilde...iredresults=16 or one of the Sealey type gas jet jobbys like this
http://www.sealey.co.uk/PLPageBuilde...iredresults=16.
The downside of the radiant one is it produces a very localised heat and might struggle to make the whole garage comfy, the other one is dearer but as it is basically a blown air heater it will distribute its heat better, obviously it is a lot dearer and they are quite noisy.
D.