Sizerace is a minigame. It became semi-official in March 2020 and official in January 2021.
Mice race to complete maps with different difficulty levels within the 90 seconds time limit. There is no shaman to help them, instead they must help themselves by changing their own size as they feel appropriate.
You can change the size of your mouse every 2 seconds. Use keys X, C, V, B, N to change your size.