The default sort order of emails in Mail App is by received date and also it does not display the size of the emails. You can follow the below mentioned steps to sort and display the mails by size.
Display size of the email
Click the View menu then navigate to Message Attributes and tick size from the menu list.
Now the mail app would start displaying the size of the email below the date.
Change email sort order to size
Mail users can change the email sort order by clicking Sort drop down and picking Size from the list.