How thick should metal table legs be?

Stiffness comes from the section shape and the welded joints, not from the sheer weight of steel. We weld table legs from closed S235 steel sections, for furniture most often 60 x 60 x 2 mm, which look slim and carry a thick top with confidence. For a longer table, a heavier top or bar height we size the section to the design and state it in the specification before production. See why section shape matters.