One important thing ist forgotten most times, when building something that has to go in a case.
You have to work backwards and think about everything else all at once!
Select the connectors and think about how you will mount them, directly to the PCB, screwed to the case, loose wires and so on.
Search for a box wich will fit them and the electronics.
Search for buttons, switches and everything else wich will fit in the box.
Then decide how big the PCB has to be, this is related to part one.
Then try to fit everything together and make it look good.
If you are stuck with a pre-determined board size, you can go two routes.
Either search a box wich fill fit everyting nicely related to the board size and then start routing and part placement, this can work very well too!
Some PCB fabs will sell the "euro board" at a reduced price for example, at least that was my experience.
There are many enclosures out there dedicated to this 160x100mm standard wich work very well and look decent.
Or just pick the biggest, cheapest box you can find and trow everything in there and call it a day