A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear kimberly,
Good day! Thank you for posting your query in our community.
Per your description, you applied Mailto Json Function to the multiple line of text column in SharePoint list. And now you can only mail the form when the amount of text entered in the 'multiple line of text' column is slow.
From this official article: List and library column types and options (microsoft.com), we can know that a Multiple lines of text column can store up to 63,999 characters.
As you mentioned, you have formatted the column with JSON Code, since it is related to the site customization coding, you can go to sharepoint-dev - Microsoft Q&A, the members and engineers in this community are more experienced in this area, so my suggestion is to post your problem in there. You will get a more detailed and professional answer to your problem there.
Thanks for your understanding.
Sincerely,
Jazlyn | Microsoft Community Moderator