The crop filter isn't the best method to crop. Instead, first select the source so you see the red borders of the source. Then press and hold the ALT key and drag the borders with the handles to crop. Cropped borders turn green. After that, if you want to get rid of the black space around the cropped source, either right-click the source->resize output (source sice) to resize the canvas (thus the video size) to the current size of the source. Or right-click the source->Transform->Fit to screen to upscale the source to fill the current canvas. The last variant isn't preferrred, because it's better to record the native resolution of the source and let the media player do all resizing.
