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Instagram usernames can only be up to 30 characters in length, and to be valid must be made up of any of only letters, numbers or underscores. (source)
This page lists random Instagram username suggestions, all of which are 30 characters long or less.
If you want to use your own word in your username, you can use the username generator, enter your own word and set the "length limit" field to 22, and then click on "get usernames".