I have to help a person, using KDE 4.9.3, and I'm not an expert on KDE.
He has not a good sight, so I grew up the character fonts on the system (from the KDE configuration tool), so he is satisfied for the appearance of Dolphin or the desktop.

But what's about Gimp, LibreOffice or Firefox, the character fonts still undersized ! Is there a way to correct that ?