If the beam across the alleyway entrance is made of straw or reeds (which are not strong enough to support the half-brick), we view it as if it were made of metal. If it’s bent (so that the half-brick can’t rest upon it), we view it as if it were straight. If it’s round (again, so the half-brick can’t rest on it), we view it as if it were square. Having a circumference of three handbreadths is the same as having the width of a handbreadth.
The two vertical poles (discussed in mishna 1:2) must be ten handbreadths tall (about 30 inches) but their width and thickness can be any amount. Rabbi Yosi says their width must be three handbreadths (about nine inches).