Twitter has allowed users to increase the size of their display names to 50 characters from 20 earlier.
In a tweet, Twitter said "Starting today, your Twitter display name can be up to 50 characters in length! Go ahead, add that middle name or even a few more emojis."
The display name is different from username. The username appears in profile URL and is unique to the user. It's used for logging in, replies, and Direct Messages. There is no change in the limit for usernames. It is still only up to 15 characters long.
Display name is a "personal identifier (sometimes a business name or real name) displayed in your profile page and used to identify you to friends, especially if your username is something other than your name or business name."
How to change display name:
Sign in to twitter.com or open the Twitter app.
Go to profile.
Click or tap the Edit profile button.
That's it.