Dear respected Alison,
Welcome to our community.
Firstly, I feel sorry for this confusing situation it happened on your side. But community moderators here don't focus on Copilot support so I may not provides you with the appropriate explanation to this.
Here is the related Copilot Community link: Copilot for Microsoft 365 - Microsoft Community Hub
However, I tried to search online and here is what I found:
"in the More Creative and More Precise modes the limit was 4000 characters, while the More Balanced mode was limited to 2000 characters."
Even better, in this month, Microsoft Copilot is rolling out an update to Edge Copilot, that increases the character limit for user prompts. And the 2000 and 4000 from your concerned part will go to 4000 and 8000.
But still, to confirm the detailed information of Microsoft Copilot, we sincerely suggest you post a new thread in the related community to get the appropriate information from the support engineers and experts there.
I appreciate your time and effort on checking these above and thank you for your patience and cooperation.
I hope that you are keeping safe and well!
Sincerely,
Mia | Microsoft Community Moderator