dimensions

How can I make obs record a selected region of a window and set the canvas size appropriately? I want to record an area of e.g. a LibreOffice Writer window as I type. The dimension of the output video is to reflect the dimension of the area I selected exactly - no transformation should be...

I'm sorry but my english is not that good enough to find what im seeking. I have to setup a livestream of a ballgame. Hardware is working and the switching between the angles works too. I have a restriction from the platform i have to stream at, that the Moderation screen for Picture in Picture...