Captivate 2019 - resizing the red screen capture window

You can choose for a custom size, instead of choosing a full screen recording. Can you show the setup before recording? 

Are you initiating this capture session from within an existing CPTX project file?  If so, the capture window is automatically set to the same height and width (in pixels) as the project itself and cannot really be changed.

Conversely, if you trigger the capture as a separate session and make the window sized and placed where you want it, then you have the problem of inserting it into a CPTX that is of a different size.  They would not be compatible.

So the first thing to work out is what your end project height and width should be and then make sure you capture at exactly that same size so that you can copy and paste the captured slides into your projects.

If you are capturing the contents of a browser window and do not want to include the address bar, then you will need to reposition the capture window so that the top corner is in the top corner of the content part of the browser and then adjust the other side and lower edge of the browser to match the red capture rectangle.

Some years ago I wrote a post about how to use a free tool for this type of work:

https://www.infosemantics.com.au/sizer-an-essential-tool-for-adobe-captivate-developers/