The NMA will need engines which are larger than the single aisle engines for Airbus' A320neo and Boeing's 737 MAX. But they will be smaller than the next size up for modern engines, the GEnx-2B for Boeing’s 747-8.
Figure 1. The NMA takes more and more the shape of a 767 replacement (A United 767-200). Source: United
This means the NMA will need new engines, at least 50% larger than the present engines designed for A320neo and 737 MAX.
An NMA engine will be sized by V2 safety speed or Maximum Continuous Thrust (MCT) criteria.
The normal Top of Climb (ToC) sizing point will be less stressing for a twin engine airliner like NMA.