The only way to achieve this would be to force a crazy resolution on one of your monitors

For example, let's say you have 2 square monitors, both running at 1000x1000 resolution (just to make this easier to demonstrate). One monitor is 15"x15" and one is 20"x20". The 15" monitor would have a DPI of 4444 and the 20" monitor would have a DPI of 2500. Let's assume that both monitors are running at their native (max) resolution. The only way to make both monitors appear to be the same and have everything line up perfectly would be to run the 15" monitor at a resolution of 750x750. This would bring the DPI of the 15" monitor down to 2500, the same as the 20" monitor.

This is the only way to achieve exactly what you're looking for. You'll have to work out the DPI for both monitors, then figure out a resolution for the smaller (physical size) monitor that brings the DPI as close as possible to the larger monitor.

I hope this isn't too confusing, but there's no easy solution, sorry.

Sub-pixel interpolation would still be effectively scaling one of the monitor images to match the DPI of the other monitor. Either way, it's not going to look pretty on an LCD. Another way to do this, would be to vertically centre your smaller monitor against the larger monitor. That way, at the middle of the monitor it would be lined up perfectly. As you get further away from the middle it would skew more and more, but without any kind of scaling this is probably the best you can get.

There are a few different aspects to this. The bezel removal you're asking about, are you referencing bezel correction for the wallpaper images? If so, this is in the works. If you're asking about aligning monitors with a different DPI, that is planned for the future with no ETA yet.

Yep! You can actually already scale the wallpaper on each monitor using the settings in the Adjustments tab.

Regarding the mouse, how are you envisioning that would work? Would DisplayFusion do a calculation to determine the DPI based on the physical size and resolution of the monitors, and then force the mouse to jump to the "correct" position when it crosses to the next monitor?