Regarding the size, I built panels around 2'x4' rigid insulation and found them to be too big. GIK Acoustics' panels are made from the same stock but cut down so the final size is under 2'x4' and that's a better size to handle/hang/look at etc. I had a couple of those too.
Two piecing the panel should be done with top and bottom pieces of the same width and with the bottom being taller. Look at a panel door or Wayne's Coating for design ques. One 2' tall panel at the top going all the way around creates a T-intersection that I don't recall seeing at work so I would avoid that. I want to use the term 'colliding planes' but am a little afraid that somebody who really knows what that term means might take me to school. Leaving a 5/8" reveal in between panels looks sharp if you can get 'em straight. Going with a bigger (6") reveal is considerably more forgiving of inconsistencies.
