LCD monitors have a fixed number of little rectangles called pixels (usually squares) that light up with different colours in order to make pictures. Each rectangle is made up of a red, a green and a blue smaller rectangle. You can see them all with a magnifying glass.
The resolution of the screen is the number of these multi-coloured rectangles in horizontal and vertical direction. That is why an LCD monitor only has one resolution it works with. From what you say, this appears to be 1920 by 1280 for your monitors.
Likely, the monitors have embedded electronics to rescale a number of different sized images to the correct resolution on the fly for you, which gives the appearance of working with a few other resolutions. This works very well for integer scaling factors. Some can tolerate it when the picture is scaled up, which means new pixels are introduced between the ones that are there on purpose. It works really badly when the picture is too large to display directly, especially if text is to be displayed. In part, this is because parts of letters disappear, lines next to each other are smudged together without space between, slim lines can just disappear. You really don't want that, especially not for extended time.
If the problem is the screen elements being larger on one screen than on the other, I suggest you solve the problem by placing the larger screen further away. This also lets your eyes focus on different distances, which some prefer for long sittings.
If you just want screen items to have a different physical size than they currently have, you could look into changing the display scale (often referred to as DPI, which is the unit). 96 dpi is the traditional value chosen, and 120 dpi is commonly available and will make screen items appear larger. I'm uncertain if different DPI settings between monitors is possible, though.
Adding custom resolutions under Xorg is done with the
xrandr command. You might need to generate modelines with