Hi S_Pofcher,
For the default margins for reading emails and sending emails you mentioned, if you mean the size of window of reading or sending emails?
Generally, when you open a new window to compose a new email, except what Katsa Rogers mentioned, you can drag the window margins (put your cursor on the margin) while you see the horizontal arrow. But it cannot be set as default, you need to change it every time you create a new email.
For the received emails, if you have set Reading Pane as Right, you can also drag the margin of reading pane to resize it. The change is applied to whole Outlook client. If you have not set Reading Pane, you can also resize it as sending email window.
If you have different scenario, please provide us an example for your request.
Thanks,
Iry