See the pic:
(http://simutrans-germany.com/files/upload/Bug_v110-0-1_r435_DisplaySettings.png)
Dear Seeker, please, download hot nightly (http://nightly.simutrans-germany.com/) - there it is corrected very well - with drag and pull corner as is on minimap and so on. So you can improve width of this dialogue as you need.
@VaclavMacurek, thanks, can workaround via keyboard, just wanted to report it.