The side board serves a structural purpose as well. The top of the bench is relatively thin, about 2". The side board makes up for this, transferring the force of planing, chopping etc. anywhere on the top to the legs and floor. It also serves to anchor what Nicholson calls "transverse bearers" that support the bench across its width like floor joists. It's not apparent from the engraving, but from the text there is apparently a narrower side board along the back of the bench to hold the other end of the transverse bearers. The result is what we call a torsion box. Clever design substitutes for mass in this bench. Perhaps a reason is that it is a joiners' bench intended to be transported to worksites rather than remaining in a workshop.
The dimensions of the bench are given as 10 to 12 feet long, about 2 feet 8 inches tall and about 2 feet 6 inches wide.