From Test-Scratch-Wiki
Pen size is a value given to all sprites. The default is 1 (a single pixel in the Scratch program), but can be changed with the pen size blocks.
Use in Scripts
Pen size can be used in animation, drawing, and much more. To change it, you raise or lower the value. In Scratch 1.4, there are an infinite amount of sizes. The Flash Player places a limit at 256 (28 possible outcomes of a byte), however. There is also a bug in the Flash Player where size 1 only draws half a pixel, and size 2 is a single pixel.
Related Blocks
A sprite's pen size can be altered with these blocks, all found in the Pen Blocks palette:
— This block will set the size
— This block will change the value