What is a good propotional height of tall hedges to square footage of a small backyard about 1900sqf.?

Question:The backyard is surrounded by 2 story houses. 2 houses on each side & 3 houses on the backside with their ground level starting at about 4 feet above ours.

Answers:
about 300 sq. feet. Maybe a 100 foot hedge, or two fifty foot hedges on the sides.