There may be better ways to do it, but this is how I do it.
My phone takes 3840 x 2160 videos.
First make sure Settings > Video Mode is not on Automatic. Use a setting of your choice, 1920x1080 at 30fps for example.
Then import your video in Shotcut.
When you move it to the timeline, apply a Size, Position & Rotate filter
Enter the size of your video in the two Size boxes
Using the filter’s settings, you can now move the video, resize it and rotate it by editing the parameters, or you can do the same directly in the preview window (personally I prefer that way). If the round handle tool in the preview screen is out of reach, use the zoom button bellow the preview screen to make the preview smaller.
Hope this helps
Sorry I can’t attach screen captures to these instruction for now, I’m not at home.
EDIT: Actually, here’s two screen recordings from another topic of the forum.
In the first one you have a demo of how the Size, Position & Rotate filter works.
Like moving the image by grabbing the round handle in the middle.
The second one is almost the same, but it show that instead of having to use the round handle to move the image, you just double click on it to be able to move the image by grabbing it from any point in the preview window. It also shows that you can use the mouse wheel to resize the image (up or down):