Welcome to the forum.
If you have the premium tools, you can select the line tool and input a single line cut that you make the length you need.
If you don’t have the premium tools you will need to create a cut line in another program or upload the shapes template file and use the line segment in the file. Here is a link to the file: Cut Shapes Template - #10 by dwardio
If you are trying to place an outline around your project that is something else we can help you accomplish.