That is nonsense. Portfolio images published on UpWork profiles are clearly visually wider than they are deep - before you even apply a given unit of measurement!
300 x 300 in any unit of measurent of a given area – be it pixel's, mm's, metres, kilometres, nautical miles, fathoms – whatever – would, obviously, make the space....wait for it....square!
Please refrain from contributing if you do not have the answer – it wastes everybody's – and more importantly – my time.
300 x 300 for portfolio images definitely DOES NOT work - unless you want the image cropped really badly top and bottom. (screengrab attached).