InMail messages can have up to 200 characters in the required subject line and up to 1900 characters in the body.
As you compose your message, a dynamic character counter will appear in the bottom right section of the composition box to let you know how many characters you've entered.
Here's a tip
Candidates are more likely to respond to shorter messages. As your InMail gets longer, the character counter will change color and encourage you to keep your message concise.