Click the green flag, then use the sliders to change the size of the ellipse.
Scratch does not allow sprites to be 'stretched' (resized independently along X and Y axis). For an ellipse, we can fake it by using the 'fisheye' effect. This trick has its limitations: width and height cannot be too far apart, the result is not always perfectly elliptical, and it doesn't work well for outlines. But the code is short, simple and fast. This project has only 24 blocks of code and a single costume (made using Scratch's vector editor). I got the idea for this project after receiving an invitation for @s_federici's studio "Simplest ellipse drawing algorithm": https://scratch.mit.edu/studios/29609673/