Shotcut does not have a simple cropping function like that as it is designed for working with multiple sources each with possibly many different sizes/resolutions. There are other tools that are designed for single files like converters that do that. Shotcut works like a desktop publishing program with a page size or a drawing program with a canvas size. But, you can accomplish it. After you determine the final size you want you need to use Settings > Video Mode > Custom> Add… to change the canvas size and then Crop.
Given the way it works, it is not easy to determine the output size by cropping within Shotcut. Instead, you can open the video file in Shotcut, choose File > Export Frame…, open the frame in an image editor to determine the output size, use Video Mode > Custom > Add… in Shotcut, re-open the clip, add the Crop filter, adjust, and Export. You might find the “Center” and “Center bias” options of the Crop filter handy in this workflow.