Now, if it turns out that the contractor doesn't see a big advantage to going with the building kit, I might just increase the width by 1 foot (13x24), and return to the previous version, and "call it a day".
I'd definitely check with the contractor, Ken. I believe that most building kits merely contain enough raw materials to build the said structure. At least some of the lumber must still be cut to size/shape by the assembler.
I've actually built a few garden sheds (the last one really nice!) in my younger days when garden shed kits were also available. However, I ended up saving a fair amount of money in buying the lumber and all other materials and paints separately instead of buying a 'kit'. I didn't like the way the kits were built (there were built samples available to look at), they look pretty on the outside, but the construction methods and hardware were cheap on the inside. I built my shed a lot stronger and a lot nicer with much better hardware than the kits came with, and still saved money. Can't remember exactly how much I saved, but it seems like it was a good 30% or more at the time.
That's been about 25 years ago now, but the same may still hold true these days. Like I say, check with your contractor first. That extra foot would be worth its weight in gold if so.