I am trying to make the border thickness on the circles in this project stay the same width without resizing when the sprite's scale is increased. How can I do this?
https://scratch.mit.edu/projects/1352925046/

Consider as an example, a sprite with a width of 12 pixels at 100% size. If you wanted to make it appear to have a width of 15 pixels, you would set the size to (15/12)*100 = 125%.

set size to (((target width) / (original width)) * (100)) %

If you want to specify a fixed number of pixels to add to the sprite, you can calculate the target width differently:

set size to ((((original width) + (extra width)) / (original width)) * (100)) %

This should be enough information for you to fix your project.

—

Although I do ask why you need to stamp the costumes as a dot can be drawn just as easily:

set pen color to [#800000]
set pen size to ((original width) + (extra width))
pen down
pen up
set pen color to [#ff0000]
set pen size to (original width)
pen down
pen up

True, I could just use pen to do that. If I do, I can use a hacked block to make it accept hex, and have it be customizable. Thanks for the help!