Hi Harish,
Your question is not very clear to me, I think you are asking about how you can maintain the size of attachment (may be spool or other document) transmitted through email.
To do that there are two ways:
1. Tx SCOT->Outbound message -> Settings -> Size restriction for outbound messages -> Further
You can maintain the size of outgoing email.
2. You can go to tx SM30, table/view: SXPARAMS and you can find the parameters controlling the size of email.
The above change is dynamic and you do not need to restart the system.
You can check the knowledge base article 1891091, in case you are facing any similar issues in Tx SOST.
Let me know if it helps.
Regards,
Sujit Kumar Banerjee.