The Visual Studio MVP Kent Bryant shares that the two primary issues that limit Git repos are huge files and huge number of files - Git’s 2 Biggest Issues . As a best practice, you should consider keeping the Git repo size under 1GB and the overall file count less than 250k files.
Here is a snapshot that compares Git with TFVC - the version control system in TFS.
Kent also shares the 'Dos and Don'ts with Git'.
When should you create a new Git repository, and when should you use a folder within an existing git repository? Here is Kent's opinion on this topic: When you should create a Git repository.