Plans can be “nominally” resized, not infinitely resized. The B section is not resizable simply because any resizing would complicate the angles and distort the intended shape of the B section. This section is intended to be fixed in size with the leg section A and C resizable to fit available space.
What many people do when modifying this design is to either eliminate either the A or C leg completely or add simple “wings: using the framing method demonstrated in the plans. Take a look through the gallery for some great examples.