Oh, another great shed model, one that I'm intending to build at some point!
Do you have a copy of The Marlow Branch? (Karau and Turner, Wild Swan). There are several photos containing the Bourne End shed. In particular, p79 shows the side quite well. There were only 4 abutments, not 6, so the spans (and windows) would have been wider. The middle span did not appear to contain a window, there were only 2. Looking at the track plans in that book the goods shed is almost square - 90% wide as it is long.
The shed at Bourne End originally had an arched entry over the track (1890s, pg 35, also reference RHW:07725 at https://swop.org.uk/swop/swop.htm ) but was sometime later squared off (pg 79). Both road entrances were arched.
Marlow had a shed too (also in the book). Of note, it would have been built about the same time that Bourne End moved their shed, but was of a different design - side entrances for the carts. Track entrances were arched, although the wood panelling there ran vertically. Otherwise, similar style - vertical barred windows, office on one end.
Another detail - the central brickwork columns have easements cut out at platform height. You can see that clearly in the Marlow photos (pg130), but none of the Bourne End photos show the track entrances that low. It seems a bit of a kerfuffle - they could have simply built the shed slightly wider and pushed the platform out a little, so I wonder if that was an alteration to allow something specific to some particular rolling stock to pass. Folding ladder or ramp perhaps? One photo in the book does show the Bourne End cart entrance, which also appears might have that detail (pg85), so that's a little confusing. That could also be a shadowy illusion in that particular photo. I can't be sure.