Prelude: I've searched it for days here and on egosoft forum. Only "this is no possible" was found.
So I started investigating modding, and replaced main .lua files with my corrections to enlarge UI elements -- and it worked, after some mistakes and fixes. But after deeper learning a simple thing comes up that allows it all without changes for each menu.

The fix: just add <uiscale>2.0</uiscale> to config.xml in next to your saves (e.g. C:\Users\<username>\Documents\Egosoft\X Rebirth\<SomeRandomId>\).
Now most of the smallest UI elements will be scalled up 2x, it applies to conversation and comms menus, and to the object rectangles, distance text and mission hints.

So this scalling applies to those items that were most hard to scale when I did it manually, BUT it does not apply to Options Menu (not a big deal), to Main Menu (this is bad, but I have a manual fix for this) and for some info table in the top right corner (I'm not sure but this is probably Event Monitor, and should be quite easy to fix manually).

If this is helpful to anyone (having that big mod community, I'm almost sure this must be known to modders, but why it can't be googled??), I can share manual modifications for those menus as well. Also there is a way to enable UI Scaling menu in the game, though it does not give any benefits comparing to just modifying the config file (which is easier actually for non-modders).

I've posted in general discussions and here, as this would require modding for certain menus.