Yahoo Mail account provides option for the users to display the size of an email. By default if your Yahoo Mail does not display this column then you can use the options link to change this behaviour.
On the right hand side of the Yahoo Mail account, click the drop down arrow available next to Options link
Select Show from the list of available menus and then followed by Selecting Size Column.
Now the email list would include a new column which displays the size of each email as shown below.