Answer: You can shear your redwood trees to keep them at the height you desire. While it is not generally recommend that trees be topped, in this case, if you shear them all around, including the top, the plants will take on a shrubby shape rather than try to grow upright as trees.
The only tall privacy screen type plant I can think of that will fit in the site you describe is Thuja (arborvitae). Thuja occidentalis has an upright, open growth pattern, eventually reaching about 30' tall and 2'-3' wide at the base. Thuja plicata, or Western Red Cedar, can reach 30'-50' tall. It has a very dense, narrow, erect growth habit and can be sheared to maintain a width of about 4'.
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