The beams in this design are 8"x8" so you should not have a problem widening the building a little. You also have to take into account you second floor system. The plans call for 2"x joists which will take up some of the knee wall hieght. The alternative would be to use 4"x4" joists in pockets, which is what the book recommends. I assume that 4' of post above the tie beams can take the outward force of the roof but maybe someone with a little more experience could chime in. Increased width would offset the increased kneewall height of the shed, which is quite tall in proportion to its footprint, especially condidering you are also going to increase the length.

Dave