QT provides a tool called "QT Configuration" which can be used to set such things as fonts and widgetry for those desktop applications using QT. KDE has the equivalent built in AFAIK. For non QT desktops, the configurator can usually be found in the distro's repositories. In my experience, installing and using it can make a substantial impact on how a QT app appears on a non-QT desktop. The package name will vary between distros. For my Debian unstable system, it is currently named qt4-qtconfig.
Edit:
Laz's graphics unit has a record that provides the actual monitor DPI if it is reporting it via EDID and the OS is passing it on (not always a sure bet).
ScreenInfo.PixelsPerInchX
ScreenInfo.PixelsPerInchY
If the monitor isn't reporting it or the OS is not passing it on, you will usually get 96 DPI. If the OS is not providing any DPI info at all, Lazarus will return a default of 72 DPI.
The screen on the laptop I am writing this on returns 118 DPI to my Laz application, which is accurate.