I do cakes as a hobby so I am still buying supplies and gear. My childhood best friend has asked me to make her wedding cake. She sent a photo of what she wants and the details don't seem to be more than I can handle.

She wants a three tiered square cake with the center tier on the diagonal. It needs to serve between 100-120. I do not have any square pans so I plan to order some, the only problem is I can't figure out what sizes to buy so that I will get the right amount of servings and the center tier will fit diagonally on the bottom tier with out hanging over the side and/or looking "off". Can someone help me?