I believe that part of the issue is that decision makers tend to think that bigger players are more durable. Might not have anything to do with velocity or pitchability, they just think they will last longer and be more durable over a long MLB season. It would be interesting to know the injury rates for the shorter vs. taller pitchers.
Another point to consider is the release point of a 6’4” vs that of a 5’10” guy. In most pitching corners, the correct % of stride length, compared to the height of the P, is 87% of the pitchers height. Using that %, a 6’4” P’s release point is approximately a foot closer, actually 9”, to the hitter when the ball is released than the 5’10” guys release point. 90mph traveling 59’6” will arrive sooner than a distance of 60’6”. Simple math. Visual Velocity. Now, add the “plane” advantage to the taller Pitcher….. combined with the possible durability point above, IMHO that’s why coaches prefer tall(er) pitchers.
Having said that, if you're putting up 0's, I don't think the coach will care if you're 5'nuthin'!