Depends on many factors (type of dish, exposure to wind, the construction of the wall...)
I have a single-brick construction on my shed wall, and a 1.2m x 1.35m steerable Gregorian mounted on a pole boldeted to (actually THROUGH) the wall. I have strengthened the mount by fixing plates to the inside of the wall at the position the bolts come through. Any bigger and I would have been risking a damaged wall in high winds...
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