Thank you for your interest in Sump Pumps Direct. The discharge on the Zoeller M53 is 1-1/2" and as such, you should resort to that same size piping to achieve optimum performance and pumping capacity. If you must reduce the piping, I don't recommend that you go any smaller than 1-1/4". Reducing it further will only create more backpressure on the pump which will reduce the pump life, not to mention the reduction in pumping capacity that you would realize.
The check valve placement can be anywhere from the discharge port of the pump to about 2-3 feet above the sump basin cover. The latter is my suggestion as it also allows for a good place to gain access to your system for service.
If you have additional questions, please let me know.