Ok, this I find very interesting ...
I asked, "Why is Optimus Prime a long nose Peter-built model rather than the classic flat-nose version?" Bay responded explaining that the flat-nose version would not allow Prime to realistically transform into the 28-foot tall robot they were looking for. The classic look gave Prime a height of only 23 feet, while the longer nose gives Prime more to work with when transforming. This height difference was a factor in keeping Megatron and Prime in the proper scale to each other.
Damn, if I'd known Bay wanted a TALL Optimus I would've sent him MY design! Its based on a Peterbilt VERY close to the movie's, (MINUS the flames!) and is surprisingly faithful to G1 OP. I actually went to a Peterbilt yard with my measuring tape, did the numbers, and mine came out at 36 feet, give or take about 2 feet.