So the length is 4*28=112mm The width would be calculated like this:
you draw an equilateral triangle in the middle with sides or 2r(56mm). The based on pythagorean theorem u know that h=rad(56^2-28^2)=48.497 28+28+h=104.497
A "cross section" is basically the two-dimensional face that you get when you cut a three-dimensional figure a certain way. For example, imagine you're buttering your bread. When you cut a pat of butter off, what shape is left on the butter? A square!
So, imagine cutting a ball in half. What shape is left? A circle!
