“I have written briefly” [I Peter 5:12]

“I have written briefly”. Then it’s unlikely your letter will bore the reader. Your letter will deal with the things that matter. Because brief, it will most likely be read.
What you say will not be so long and complicated that the reader either doesn’t know where to start or forgets half you’ve written. Lots of people will be made glad if you write briefly and because you’re brief you’ll have time to write to lots of people.
Now if you’ve never written a letter of encouragement, or it’s so long ago you’ve forgotten, get the ink flowing through your pen again. Look around you: lots of people need encouragement: life’s quite hard for some. They’ll appreciate the letter you write, reading it again and again, delighted you’ve taken time to consider them. And do keep it brief.