Build my own pc

Solution
There's nothing wrong with using an ITX board with an ATX case. However, that said, you won't get use of the full functionality of your case. If you're OK with that, then you can proceed to use it. Better would be to return the motherboard and get the ATX version if you possibly can.
Can't make any recommendations without knowing what you bought. Dimensions aren't exactly that useful.

Plenty of motherboards out there with Wifi, but with larger boards it is simpler to buy a replaceable WiFi card.

So do you want a small computer or a large computer? What do you plan to do with it?
 


Oops, i forgot to post the link...
 
There's nothing wrong with using an ITX board with an ATX case. However, that said, you won't get use of the full functionality of your case. If you're OK with that, then you can proceed to use it. Better would be to return the motherboard and get the ATX version if you possibly can.
 
Solution