One quick thought to consider: If you want to make the FlightGear display "lifesize", then set the field of view to match the exact field of view the monitor actually covers. This means pulling out a ruler and measuring how far your display is from your eyeball (careful not to poke yourself in the eye.)
Then measuring the side to side length of your display and doing a small amount of trigonometry. If you get this right, then everything that FlightGear displays will be life size and any distortion effects will be minimized. You can apply this same basic strategy with multiple monitors and match the angle offset of the center of each display to what it really is with respect to your eye point. The goal is for straight lines in the sim (like runway edges or the horizon) to project straight from the end of one display to the start of another (i.e. no stair-stepping in banks.) With a small bit of attention to detail and some fiddling around, you can make an awesome and very immersive multi-monitor setup with FlightGear. Long ago when I was a driving simulator engineer at the U of MN, I installed flightgear on our driving sim visual pc's and tested it out:
https://www.youtube.com/watch?v=gwl7OS40o_E In this case it was one PC per display because that was how are hardware was setup, but flightgear also supports multiple monitors connected to a single PC. I know this has been tested with at least 8 displays connected to a single PC (probably even more at FS Weekend.)