GameRager: Questions: What do I set the number for 2d overlay to for, say, 1366x768? What number would be good? (I mean mine looks ok but is currently a tad small)
There are two separate options:-
d3d_disp_scaled_2d_overlay x scales in exact multiples 1x, 2x, 3x, etc according to percentage (the lower the number, the larger the UI). Obviously there has to be enough space on the screen to fit (it won't "overscale" to the point where the UI will get cropped / cut-off). On my 1920x1080, 64 is twice the size of 128 but 16 isn't any bigger than 64. For 4K, 16 is bigger than 64. For 1366x768 I don't think it scales any more using this method and you need the second below option.
d3d_disp_scaled_2d_overlay x y is the 2nd method where you can define a fixed resolution that gets scaled and aspect ratio corrected vs your actual resolution. Eg, if you find 1366 x 768 too small, then comment the ;d3d_disp_scaled_2d_overlay 128 line and uncomment the one immediately below it and use the figures
d3d_disp_scaled_2d_overlay 800 600 (slightly larger) or
640 480 (quite a bit larger). This doesn't mean the game will run at that (it'll be native 1366x768), it means it will enlarge the UI in a way that's calculated to be the same equivalent size as if you were running at that resolution.
GameRager: What does the bilinear resizing thing do?
Bilinear resizing means it will smooth the UI out when it resizes. This allows it to scale to any fractional factor. Think of how photo editors work when you enlarge it and tick a high quality scaling option. The other option is "pixel perfect" / nearest neighbor which will simply double the width / height of pixels without smoothing, it appears less blurred and more blocky and is limited to exact multiples 2x, 4x, etc, but some people prefer that look, eg, 4k vs 1080p is exactly 2:1.
GameRager: What is a good FOV/best FOV for 16:9/1366x768?
It's really down to personal preference, how big your screen is, how far you sit from it, etc. If it feels too claustrophobic, increase it. If it feels too "zoomed out", decrease it. 80-90 is what most people usually game at on PC's.