GRSize от pluvivanto

A simple extension that adds a repository size breakdown to the GitHub sidebar, so you have a better idea of how much you're about to clone.
GRSize doesn't show ads and doesn't collect any user data. It is fully open-source https://github.com/pluvivanto/GRSize

Note: The numbers come directly from GitHub's API. Expect 10-20% increase in actual disk usage due to filesystem block allocation and Git metadata.