What are the criteria for conceptual design of thrust reverser in the use of braking?
As @John's answer alluded to, the rules for actual landing distance (25.125) and the accelerate-stop distance on dry runway (25.109) effectively prevent a manufacturer from taking credit for thrust reverser (T/R) when demonstrating/calculating for these distances. However, 25.109 does allow a manufacturer to take credit for T/R when dealing with a wet runway. This means that having a (good) T/R gives an operator more flexibility when dealing with adverse weather conditions, and this translates to more money saved (vs sitting on the tarmac).

Therefore, just like any other performance numbers, the performance of T/R is subject to market objectives. There is no regulatory requirement on how powerful the T/R has to be at max reverse.

That being said, this NASA memo has some helpful information on why airlines want thrust reverser at all. The following two graphs are cited from this source, which outline the percentage of work done by the respective braking systems for a typical jetliner on dry and wet runways. You can perhaps back-calculate the thrust of T/R from your design point:





Other factors to consider for design:


Foreign object debris ingestion from T/R deployment, especially at lower speed
Compressor stall from hot reversed gas, especially at low speed
Inadvertent deployment of thrust reverser in-flight. For most airplanes, this would be catastrophic.
Controllability in single T/R deployment on ground
For rear mounted engines, aerodynamic interference with rudder effectiveness