I am currently having a house built. I drove by yesterday to check on things and noticed that the fascia is one size on 3 sides of my house but double the size on the 4th side. Is there a reason for this?

You're asking US?? (sorry, couldn't resist...)

Seriously, are the roof slopes and overhang dimensions the same all around? Other than those, I can't think of a reason to have differing sizes, especially double. By all means ask the builder, and if his answer seems a little tilted to you, get back to us here...someone will surely offer an opinion.