head -c 100 /path/to/file
head by default prints the first 10 lines.
-c 100 makes head print the first 100 characters.
To print the first 100 characters of the output of a command, simply pipe the command to head -c 100 without specifying a filename, for example:
curl -s 'https://api.github.com/orgs/github/repos' | head -c 100