Is it possible to get Captivate to force browser to opem maximised?

There are no default settings in Captivate to make the conten load maximised on first opening.  There is a Fullscreen option on the Publish dialog.  But this just creates an extra HTM page with a link that has to be clicked in order to open the actual module at full screen size.

If you have some control over whatever link it is that your end users click to open the Captivate HTM file, then you can use the JavaScript code from the link inside the Captivate fullscreen HTM to script the link your end users click.