@Anonymous Thank you for reaching out to Microsoft Q&A, apologize for any inconvenience caused on this.
For Send Message operation in SAP connector if you are using on-prem data gateway the max value is 2560 and if you are using Integration Service Environment (ISE) the max vale for request body is 102400 as mentioned in the documentation here.
As per my knowledge, this payload size is not extendable, I am checking with internal team on this as well.
Feel free to reach back to me if you have any further questions on this.