I found a link to a heater size calculator, its on a site that sells heaters so I didn't post it because it may be considered spam, but I selected average insulation/air leakage, required temperature increase of 10 deg. F, 12'x12'x8' room and it came up with 1,547 watts.
That will be 12.9 amps if its a 120 volt unit or 6.4 amps if its a 240 volt unit.
Either way you need to add a new dedicated circuit for it, if it was 120 volts it will be too many amps to use on a regular circuit.
I would think it would be best to install the heater on the coldest wall, like under a window, since that would be the coldest area. I always worry about heaters in bathrooms, be sure no one touches the dial when they are wet or in contact with a grounded object, even if properly installed and grounded there is still a chance of a hazard. And keep it far enough away from the shower so it doesn't get splashed with water.