Have a look here for ideas:
http://www.twowests.co.uk/TwoWestsSite/category/HEATING_PARAHEAT/I have used parasene heaters in the past, including the mini, midi and standard blue flame heaters.
The mini will keep a quite hard frost at bay in an 8 X 6 especially if you have it insulated with bubble wrap.
There are some problems with the paraffin heaters:
The smell of the fuel and fumes though you can add a scent to the fuel to reduce this.
They do release a lot of moisture into the air so condensation can be a problem so you must make sure you have adequate ventilation.
If you do not trim the wick correctly they can pump out lots of soot over your plants, I did this once and lost a whole house full of seedlings.
If you are working in the house make sure you ventilate the house well to prevent breathing in the fumes.
For these reasons I swapped to electric a few years back and find it far easier to manage. I can go away for a few days and not worry as the thermostats control things for me.
HTH
Jerry