There’s a way to access this information through the GitHub API.
- Syntax:
GET /repos/:user/:repo - Example: https://api.github.com/repos/git/git
When retrieving information about a repository, a property named size is valued with the size of the whole repository (including all of its history), in kilobytes.
For instance, the Git repository weights around 124 MB. The size property of the returned JSON payload is valued to 124283.
Update
The size is indeed expressed in kilobytes based on the disk usage of the server-side bare repository. However, in order to avoid wasting too much space with repositories with a large network, GitHub relies on Git Alternates. In this configuration, calculating the disk usage against the bare repository doesn’t account for the shared object store and thus returns an “incomplete” value through the API call.
This information has been given by GitHub support.