This topic is one continually revisted on other boards. Well meaning people state, firmly, they only keep the 12 to 16 inch "eaters' and release all above that. Others state, authoritatively, that all who eat any walleye above "X" inches make a mistake and will kill off the next generation.
The most prolific spawners are 15 to 20 inches, That does not matter in TR as spawn is not the source of recruitment. Stocking is the source of recruitment. TR is a put and take fishery. Whether you keep a fish 18 inches or up does not matter. Do not keep eaters - that violates the law. Keep the few you catch over 18 inches - good eating. And, BTW, if you think any fish over "Y" inches is not good eats, drop them off with me.