I'm 6' 5" with an inside leg of 36/37.
I ride an XLS in 59 and it is perfect. I have previously ridden the XLS in 57 and the Disco in L and they worked with stem and saddle adjustments, though they were a little on the small side.
The XLS has a shorter headtube than the Disco, for the same size so that is worth bearing in mind.
At 6' 3" I don't think an XL/59 would be too large, particularly given the relatively short headtube. If you prefer your frames smaller, then maybe consider a 57 but otherwise a 59 should do it.
Conventional wisdom used to be that you ran saddle height a cm or so smaller on a cx bike. This has pretty much died out now - almost everyone runs the same saddle height and position as on a road bike.
Smaller frame sizes potentially make the bike stiffer, but the XLS is very stiff already and the larger frame size makes shouldering on the move, and carrying easier as you can fit your arm/shoulder through more easily.