Main displays (3, 1, 4) are 27" 1920x1080 monitors, iiyama ProLite X2783HSU. These are set up in Surround at 5896x1080 combined resolution. 2 is touchscreen and 5 is touchscreen. The profile only takes up a little less than half of the #5 screen because the rest of the space can be used for stuff like TeamSpeak, HWiNFO monitoring, etc. It is landscape.
You can see the #5 screen better in this pic:
All of these are out of one 980Ti GPU and on-board graphics. I have a i5 6600K at stock clocks. My three 27" displays are powered through the GPU, the 4th GPU output goes to my HTC Vive. Main touchscreen connects to the motherboard via DVI-to-DP adapter cable. Left touchscreen connects via a Targus docking device... it is DVI-to-DVI to the Targus dock then the dock connects to the PC via USB. I have experimented with adding a 2nd GPU (non-SLI) to power the touchscreens but testing that did not prove to be beneficial, in fact, it made matters worse so I went back to single GPU.
As for using a 24" for #5 position, I think that's still possible but may need to get creative on the space. It may depend on how big the bezels are. I did not like having to cram everything onto the single monitor; there are a lot of stuff on the left AUX console that are essential for flight and SA even if all you do is glance down at them every now and again (TWA lights, Chaff/Flare count) or need rapid access to it (EMER JETT, Stores Config switch, CMDS Program and Mode buttons) so I really like the 2nd touchscreen.
Hope that helps!