[Lazarus] Detecting desktop area size

[Bart]

Hi,

Is there any (more or less) reliable way to detect the size of the
desktop (that is: on the active screen, the space a form can occupy so
that it will take in account the presence of a taskbar etc.)

I need something like this to keep a form that I create at runtime in
view on the desktop (it's initial size and widht/height) depend on the
contents it must display).

I know how to do it on Windows.
Linux (GTK2) and MAC would be nice.

Bart