This is not a puzzle. Just a very simple project ;) Because the "size" property of sprites is set and read in percent (%), I often make my costumes 100 pixels, so that "size" (%) will correspond to it's size in pixels. Note: In this project, Size (pixels) doesn't always match Size (%) exactly. In this project, I simply demonstrate this. All code by me (@kriblo).
Turned out to be quite a puzzle... I noticed that sometimes the measured size (pixels) doesn't match the set size (%). When I first ran my test, it seemed to happen infrequently, so I didn't think too much about it. But then @papipupepappa pointed it out in a comment (below), and I started thinking. I tried making a sensor costume half a pixel from the center (not used, but inside). I tried bitmap and SVG costumes. I tried measuring in different directions. So far, no luck measuring correctly. I'm taking a break from this now, but can you solve it?