I'm putting together a similar 3 projector setup and have just finished my back of the napkin calculations for screen size, etc. Here are some considerations when optimizing megapixels and aspect ratio:
1. Your calcs should all originate with the native resolution of the 3 projectors. (i.e. BHawthorne has 3 XGA projectors for a total of 3072x768 or 2.36 MegaPixels.)
2. If using sol7 edge blending,you need to take into account the pixel overlap region. (
http://immersaview.com/overlap.html) i.e. using sol7's 12.5% overlap metric with 3 XGA projectors, it will reduce the resolution region to 2816x768 or 2.16 MegaPixels.
3. Take the calculated aspect ratio from the overlapped resolution (3.67:1) and apply this ratio to your desired screen height or desired screen width.
Solving for Height Example: You want a 6 foot high screen, then you will need to go 22 feet wide (6*3.67)
Solving for Width Example: You have a 15 feet long room, so you will need to go 4.1 feet high (15/3.67)
Excel spreadsheet if anyone's interested:
https://ispace.utmb.edu/xythoswfs/webview/_xy-244646_1
One last thought in terms of megapixels, if you used 2 1080p projectors they would yield 3600x1080 after correcting for overlap, which is 3.80 megapixels and an aspect ratio of (3.33:1). These calcs are making me think about purchasing two 1080P projectors which down the road I could eventually get a third after switching to eyefinity.