We constructed a bill board sign, 24' long x 12' high which is facing North-West. We have wind gusts up 120 k/hr at times in Alberta Canada. We are considering drilling holes to reduce the wind load on the sign. What should the hole size be and how many?

You may find a better answer by posting this question in the engineering >civil engineering forum, as wind loads are commonly factored by civil engineers into the structures they design. Their approach will be to calculate a wind load and then design footings that can withstand the over-turning moment. The billboard structure itseld will have to be constructed to withstand the load as well. So basically the question of "what size holes and how many" can't be answered without going into the design of the billboard structure, supports, and footings.