I solve this eventual question and my answer is
Among the question choices are
7 x 7 ft and
7.5 x 7.5 ft. Which one should I choose? I guess if a safety margin is intended, the larger choice i.e.
7.5 x 7.5 ft should be selected. Am I right?
There are situations like this in which two question choices are close to my final answer and I'm not quite sure which one to pick. Is considering safety margin a good criterion to pick the choice? Thanks for your help!