Assuming you are asking about the CPU fan it must blow toward the mobo; air spilling sideways from below the CPU is needed to cool the chips in that area; if you use a low flow fan it may be necessary to put extra cooling on the major chip(s) in that area. I have done so.

In my setup, the fan wouldn't really be the “CPU fan”, but simply “the fan” Let me see if I can sketch that…
The outer rectangle is the case viewed from the top, case front on the left hand side, case back on the right hand side.
The square in the top-right corner of the case is the MB (naturally against the back of the case to give access to all connectors.
The small rectangle in the bottom-right corner of the case is the PSU (also against the back of the case, where the power cord is connected), and just next to it on the MB is the small 4cm×4cm CPU heat sink (letter “C” on my sketch).

My goal would be to remove both 4cm×4cm fans (the CPU one, and the PSU one, located at the extreme corner of the box, next to the power-cord connector). To replace them, I would use a big fan (dotted “circle” in my sketch) above both hot parts, fixing it along an equivalent hole made in the case cover using a hacksaw (actually the electric equivalent of it, for which I was not able to find the word).

Yves.

Understood. The requirement to blow toward the mobo remains because the other direction would result in less airflow past the chips on the mobo around the CPU. If the case is metal, hacksawing a neat hole will be very difficult.

I think that stores like FrozenCPU sell rubber trim to put around sharp metal openings.

I chose a Noctua NF-S12B FLX.
Yesterday, I removed the cover so I could see the inside, and I concluded a 14cm fan simply wouldn't fit, and would be oversized anyway. Besides, Noctua say on their site that their 14cm fan is reinforced so that it can bear with forces that appear at this fan size*; so why not avoid this size in the first place? ;-)

I'll let you know how things are when the hardware comes home.

Yves.

My computer has 2 14cm fans in it, and I think they work better that any smaller fan.

But 12cm is great, too, especially when compared to 4cm and 8cm fans.