The absolute biggest I can go is 600 square feet.
it will be a vaulted ceiling to allow for the lift to go back in.up to todays energy codes
on the cooling side normally if I was asked to cool this area I would chose a 1,5 ton unit to over come a hot car coming in the garage in summer.for my garage I will chose a 1 ton unit
on the heating side I would normally install a heater with an output of at least 33,000 btus so there unit 45 would do it.But because I want a real quick recovery in colder weather I am chosing there unit #60
Here is a link to there site
the company has been around forever
http://hot-dawg.modine.com/unleash.php?expand_index=0
Again you would not use my sizing recomendation for a home but I have noted why I am chosing to go this way
If there was a manufaturor who made heating unit like the hot dawg that used outside air for combustion I would use that one instead to avoid infiltration and and the unit would stay in better shape because it was using cleaner air then that found in a garage.
Probabley the best system I could install would be a condencing warm air furnace located in my wifes garage with a standard a/c system ( it would be safe because it uses out side air for combustion) I could also add a high eff air cleaner to clean the air.But I am chosing the fujitsu system strictley for its dehumidifing properties and because the outdoor unit is very small .a standard a/c sytem can dehumidify just as well as the fujitsu unit but I would have to add alot of controls to it to slow down the fan speed and another controller to allow the oudoor unit to remain running in colder tempitures.
I also have a boiler in my home that I could have used with a hot water hanging heater (hydro air)but for me its easeyer to run 1-gas line then to run the heating pipes over from the boiler.
When we did my partners home he chose to do his house with a hydro air set up and we do not run anti freeze in the heating pipes we just installed free-a-stats on the water lines so if the heating pipes ever get to 38 degrees they just turn on the boiler and the circulator till the tempiture of the pipe get 5 degrees warmer.this is to avoid them freezing if some one leaves the door open.
I DO NOT recomend using antifreeze in a heating system unless its isolated from the boiler with a plate type heat exchanger.
I could also install radent heat in the new garage slab but I always try and design a system as simple as possible.Allthough for my customer I do what they want my personal phillosophy is to keep things as simple as possible.