Fullscreen issue with V5.1

Fullscreen with widescreen disabled produces a true 4:3 aspect resolution screen, not a widescreen with black bars. This is a problem with your computer's display settings, not the game.

Your post hasn't really described what's actually being shown on your screen, but it sounds like whatever the issue is, it's with your graphics card and screen scaling in resolutions lower than native. If you're seeing a tiny box in the middle of the monitor with the game window when you're in fullscreen, you may not have your screen scaling set correctly. You need to make sure your monitor is correctly upscaling low resolution modes to fit your monitor as large as possible. For instance, with an NVidia card you'd go into NVidia Control Panel, go to "Adjust desktop size and position" and make sure scaling mode is set to "Aspect Ratio" (and set perform scaling on GPU in case your monitor is not handling the scaling correctly).