I recently got a laptop which has 2880 x 1620 resolution on a 15" screen. After installing KDE there, by default everything looked really tiny. What are generally recommended methods for improving UI look and feel in such situation? I can increase the DPI, but that would just affect the fonts.

Any general advice? Besides the DPI, I found some settings which affect individual widgets like the width of the scrollbars for example and etc. But going into so granular modifications is rather tedious. Shouldn't be there some global setting for the UI which addresses this? Or such resolutions are still very new and KDE didn't catch up yet?

The Oxygen GUI style should be DPI aware, but i don't know whether this holds for the Qt4 implementation as well.

On some shameless self-advertisement:

Bespin has (since the very first release) an environment variable, eg.
export BESPIN_SCALE=2

Virtuality has the same (early versions used the bespin env, later versions VIRTUALITY_SCALE) and latest versions interpret the present DPI automatically.

Some components do honor the DPI setting (for example cursor is sized accordingly). But apparently not all - for example scroll bars and window controls (close / minimize / maximize, etc.) aren't affected by it at all. Is it expected?

My case is KDE 4.14.1 so it's still Qt 4 of course.

I found this: https://community.kde.org/KDE/High-dpi_issues

Looks like KDE still didn't catch up in quite a number of components and this is going to be addressed only in Plasma next.

Some more ideas here: https://wiki.archlinux.org/index.php/HiDPI