Whether a video plays in full screen or not depends on the aspect ratio and resolution of the file you want to convert, the output settings you choose and the aspect ratio/resolution of your device.
To play videos in full screen, do the following:
a. If the aspect ratio of the source video is the one you want, please click the Settings button, choose the output format from Profile and set the resolution same as your original file. You also need to set Edit > Crop > check Keep Original option.
b. If the aspect ratio of the source file is not the one you want, please select the required resolution on the Settings window and Edit > Crop > set Zoom Mode as Full Screen. You need to crop your video first is there is originally a black bar.
c. You can click Edit > Crop > set Zoom Mode as Full Screen, and use the yellow Crop Frame to remove black bar or unwanted parts of your video.