I've only tarped with the basic blue plastic grommeted things available at every hardware store, so my size selections were limited to those available -- all rectangular.

I agree, 5' x 7' is too snug. 8' x 10' on the other hand worked perfectly as a solo shelter for me: roomy, dry, and tight enough to use in windy conditions.

For a two-person shelter, 10' x 12' was the size that seemed to work best for me.
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It's easy to be a holy man on top of a mountain.
-- Larry Darrell