Welcome to the forum. I won't comment on the engineering, I am not qualified, but you don't have to cut it down a full inch. Typically you would undersize to the nearest half inch, depending on the quality of your timbers. In my understanding you would only go down a full inch if there was a large variation in the dimension of the timbers. I am sure there will be someone along shortly to help with sizing issue. What kind of mill are you working with that will only cut 7" at a pass? I am assuming some form of swing blade mill, i.e. a Lucas or Mobile Dimension.
I am also going to be cutting one of these sheds this winter, but I am going to try and lower the overall height, it is a little to large for the location. I will probably shorten the posts above the tie beam, which is what Jack suggested to me. Good luck and stop in often, there are many experienced people here that are happy to help.
Dave
