you'll need to drill down to the resize option, so click all those things i've highlighted in the image, top to bottom, left to right.
then, choose how you want to resize the video. 4:3 or 16:9. padding and/or cropping etc.
if you are making it fullscreen (4:3), it's probably better to uncheck the "show safe area" box so you have a clearer view of the edges.
hope that helps.