I've been extremely curious about this for a few minutes now, because it kinda bugs me that 4:3 stages, if used in Ikemen rather than Mugen, will automatically crop to the game width and height if the engine is set to a 16:9 resolution. Stage fit is an extremely useful feature within specifically Mugen 1.1, and it sucks that there is quite frankly no way to do this exact same kinda thing on Ikemen.

Edit: Just to clarify, I have the game width and height in my build of Ikemen set to 3840x2160 (a.k.a. 4K), which is the exact size of my monitor. I also tried using the Force Stage Zoom In and Force Stage Zoom Out options within the config.json file to no avail.