Add Rich Text Column to Document or Asset Library

Here are the solution steps:

1. Add a new multi-line text column to you library. Make sure the "Allow unlimited length" box is selected. There will be a pop-up box. Select OK
2. Save the new column
3. Open SharePoint Designer to edit the library
4. Under "Customization", Click "Edit list Columns"
5. Find the column you just created and chose "Column Settings" from the ribbon
6. Check the "Rich text" and "Enhanced Rich text" boxes
7. Select OK
8. Save & Exit Designer and refresh the page

If you can only enter 225 characters after updating the rich text properties, you forgot to unlimited length part of step one. Update that box and then follow the steps