Lacona,
The default window size is set in your windows properties
dialog in RoboHelp. It can also be set manually in the .hhp file
before you compile. If you open the .hhp file in a text editor,
such as Notepad, you will see a [WINDOWS] section and for each
window defined there will be a series of numbers. One of these is a
vector [in square brackets] that defines the original position.
These are absolute screen pixels.
In older versions of RoboHelp, one of which I use, there was
a limit to the size of the window RH would allow but when I changed
the .hhp, RoboHelp didn't try to overwrite it unless I reopened the
Windows properties dialog. I'm not sure how the latest versions
handle this but you might try editing the .hhp with Notepad and
then compiling the .chm.
John