Iain,
There's not really anything you can do on the CHM side about
this because, when specifying the dimensions and position of the
help window, you are permitted to enter pixel values only. This is
a limitation of Microsoft's design of HTML Help.
When calling the CHM from an application, you can use Windows
API calls to adjust the size of the help window. (For example, I
think this is essentially what Microsoft does with the online help
for the Office applications.) The following page provides some
sketchy information, but I think you'd probably need to hunt around
the MSDN site for the full details:
http://helpware.net/FAR/far_faq.htm#control
Pete