Which diameter EMT for car roof rack?

Which diameter EMT for car roof rack? I saw the video of the awesome van rack and am pursuing a similar design; mine will be maximum 7’x4’ perimeter. Which size of EMT will be stronger given the various potential stresses of tie-downs and load shifting? I understand that larger interior openings are more crushable, but the larger sizes have slightly thicker walls. I am less concerned about cross-bars because they can be easily replaced. The curves and bends of the perimeter are what I want to optimize. TIA for any advice!

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