I do not believe the input has a limitation on characters. It ends up being an attachment. Why would you not just copy and paste your "attachment" into the input area?

Enter an explanation for the changes to income, deductions, and/or credits. When the program prints Form 1040X, the explanation prints exactly as you type the text (i.e., all spacing and line breaks as you entered them). The explanation prints in Part II of Form 1040X.