XCOM1 and TFTD graphics were made for 16:10 graphics. It is at 16:10 that the world in the geoscape is round. I have no idea what idiot at microprose thought that people used 16:10 on DOS machines in 1992, they were all 4:3 and as a result everyones globes looked vertically stretched.
At any rate in running OXC in a 16:10 resolution results in nice square pixels, cleaner fonts, a round globe, square base facility tiles, etc. However there are only 2 16:9 options in the options menu, and most people don't exactly know what the ratios of various resolutions off the top of their heads.
I recommend having a separate aspect ratio selection in the options and allowing the user to select from common resolutions in that aspect ratio. Also I assume that at some point we want users to be able to resize the window in windowed mode. If so there should be an option to maintain their selected aspect ratio.
An alternative would be to allow resolutions to be selected from a table. However in the mean time you can set the resolution in the cfg file.
Common resolutions in ratios:
5:4
320x256
600x480
640x512
1280x1024
1800x1440
4:3 standard TV/monitor
320x240
400x300
640x480
800x600
960x720
1024x768
1280x960
1440x1080
1600x1200
1920x1440
16:9 standard widescreen
640x360
960x540
1024x576
1280x720 aka HD 720p
1600x900
1920x1080 aka HD 1080p
16:10 or 8:5 Ideal for OpenXcom
320x200
640x400
768x480
1024x640
1152x720
1280x800
1440x900
1680x1050
1920x1200