A test strip is a crop of an image so there's a test strip button on the cropping tab in the full page editor.  Just set your print to the full/final size you want and click the test strip button until you get a piece as small as you want.  Or... click the test strip button once and after that, choose a new size which will be your test strip size.  You can move the crop around on the same cropping tab to get the exact piece you want.

Example: add a print at 13x19 because that's the final size you want.  Click the test strip button and then choose 4x6 size.  You'll get a 4x6 test strip of your original 13x19.

Regards,
Mike