As I understand it, for the moment, it is impossible to send a notification with more than 1000 characters by email, due to the apparently hard-coded text field limit of 1000. (Here’s the related RFE to increase it: Ability to have a larger Text field.) Is there any other way to send an email with a 1000+ text? Preferably rich text or HTML tags accepted?
You can create an Email Message record (Mail module). The body is a Note field, so you can map in much more text. Create the To Email Address and From Email Address records and associate them to the proper people records and trigger the Send action. This one involves more work than a notification helper, but it does allow you to send emails with larger bodies.