To resize or crop a source, make sure it is not locked (lock icon in Sources list is open). Now when you select a source a red boundary box is visible.
Resize: drag one of the little red squares.
Crop: hold down the Alt key (on some computers apparently only Alt Gr will work) while dragging a red square. The red border becomes green.
If you need pixel precize adjustments, right click the source and select
Transform,
Edit Transform and enter the desired values.
Don't forget to lock the source when you're done to prevent accidental changes.
https://obsproject.com/wiki/Sources-Guide