when I call raspivid -md6 from the command line I get a full screen picture but if I use picamera from python I am unable to change the size of the preview on screen, any suggestions? what am I doing wrong?
The output size and resolution of the preview window using picamera can be set independently, and there is a requirement that the resolution be no greater than the input resolution.
Something like this should set the output window if you don’t want it to be fullscreen:
If you continue to have trouble with this, please post the code that you are using and we can try to help.