Personally I would want the tape to cover about 3" X 6" minimum, even if the bracket is not that large. This would give 18 square inches of tape on the roof surface. If the tape has a pull strength of say 1 pound per square inch, more on the roof will cover more area, and this will be a stronger joint than just say 2" X 4".
When my instructions for my original solar system came, it said that using rubber roof sealant below the 'bigfoot' brackets can allow you to not have any screws into the roof. I put in screws anyway. The brackets are about 3" X 4".
Fred.
