I have an MCRX also. You can make a 16x20 with either a 135mm or 150mm lens on that enlarger. But you need less headroom with the 135mm, no matter what enlargement size. If you want to go bigger than 16x20, the 135mm will be the better option unless you have or build a drop-table.
Of course, the MCRX has the bracket to allow you to shoot horizontally, but from all accounts, it's difficult to achieve good enough alignment to manage a sharp print corner to corner that way. I've never tried it.
I mainly use an EL-NIkkor 135mm on my MCRX for 4x5. I have no problems with coverage.