its not about how big the fan, its about how much air it moves, for clearcoating you will need good air movement.
for a 2.5 x 2.5 a good 10" or 12" fan should work great. make sure you use a control switch for the fan, this way you can turn the speed down or up to suit. you will need an air inlet for it aswell as around the same size as without it you will cause pressure and vacuume in the room, it should be able to remove the air and replace it easily to give a good airflow through the booth. make sure you filter the inlet to stop anything being pulled in with the air movement.
20 to 26 degrees is what normal booths run at for painting temperature, this will be difficult to achieve with home made booth but will probably only be an issue for you in the winter months
infa red or haleogen heater are great to help keep the room warm, but again all will depend on how much air is moved by your extraction