Need some advice...

I was thinking of putting a 120mm fan on the side panel and was wondering if that was a good idea because I've only seen 80mm fans being used. I am new to this so please excuse me if this was a stupid question. Any help would be appreciated. Thanks in advance.

120 mm fan will do fine on the side, you might want to put one on top, i did, it really draws the heat out of the case, cause we all know heat rises.heres what i got, my back fan i have blowing in 80mm, my side fan i have blowing in 80mm, i have a dual fan PSU drawing out, and an 80mm on top drawing out, this seems to work good for me.

I think the best spot for a 120mm would be on the side panel (and a duct may be good) so you can have a wind tunnel going straight to your cpu.