https://help.github.com/articles/what-is-github-pages/
GitHub Pages sites are subject to the following usage limits:
GitHub Pages source repositories have a recommended limit of 1GB . Published GitHub Pages sites may be no larger than 1 GB.
GitHub Pages sites have a bandwidth limit of 100GB or 100,000 requests per month.
GitHub Pages sites have a limit of 10 builds per hour.
https://help.github.com/articles/what-is-my-disk-quota/#file-and-repository-size-limitations
File and repository size limitations
We recommend repositories be kept under 1GB each. This limit is easy to stay within if large files are kept out of the repository. If your repository exceeds 1GB, you might receive a polite email from GitHub Support requesting that you reduce the size of the repository to bring it back down.
In addition, we place a strict limit of files exceeding 100 MB in size. For more information, see "Working with large files."
