I'm in the process of building Optimus Prime as well, I've completed it up to part 2d. I'm getting ready to build part 3 and have parts 4 and 5 as well. This thing will be 1.4m tall when done, that's almost 4ft. I'm using photo paper as well and the problems I've ran into most is that photo paper doesn't like to glue too well. I've hit this blasted model so many times that I keep popping parts off. Photo paper is coated so that the glue doesn't get to soak into the paper, it more or less just sits on the surface.
Also because photo paper is almost twice as thick as the paper that it (the model) was designed for, some things are a bit wonky to fit....ie: holes in some of the parts are let's say 1/4in but the part going into it (because of the thickness of photo paper) is 1/8in bigger. I'll post a picture of my build as soon as I can find my blasted camera
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Rusty