Rechteck - Strichbreite

. . . Or:

• Make a selection around your rectangle shape with the rectangular marquee tool or simply create a rectangle with the marquee tool
• Go to Edit>Stroke (Outline) Selection . . .    This will bring up the Stroke dialog
• Enter your desired pixel width  for the the Stroke (and choose from the other available options for color and position)
• Click OK.

Hey, @sv_grube. Welcome to the Photoshop Elements Community. I'll need more info to help you figure this out. 

Based on the limited description, it is unclear whether you wish to resize the rectangle or add a stroke to the rectangle shape. 

You'll have to switch to Advanced mode to apply these changes.  You can resize the rectangle in the tool menu.