Genuine Pella Replacement Roller Assembly for Pella Sliding Screen Door. 

Overall Height: 2-3/16 inch
Top Width: 1-3/16 inch
Base Width: 3/4 inch
Roller Diameter: 3/4 inch
Material: Plastic 
Qty: Pair (2) top rollers and (2) Cord and Screen Holders
Installation Instructions: Included with kit
Tools Needed: Phillips screw driver and pliers
Possible Markings on some: 777K / 777L 

Note: 
The screen door rollers are located solely at the top of the screen door. You need to remove the screen door to replace the roller.