5:41 PM, Sunday January 10th 2021
That one is triky because it requires that you have more knowledge of perspective grids which he talks about in the next chapter. Try jumping to P 51, which is the one that teaches you how to make the right grid for the exercise. In the video for P 37 he explains the process more throughly: http://scottrobertsonworkshops.com/h2dr/videos/
Regarding the right size, the planes are theoretically infinite. When Scott defines a plane size in the book it's merely for illustrative purposes, plane size doesn't come into account when mirroring, maybe this is the reason for your confusion? Regarding the angle, the only requirement is that it's vertically oriented, like the side of a 2-point perspective cube, and it's also best to keep it close to the tilted rotated plane you're going to mirror.