In Mail Editor, you can use the Zoom controls located at the bottom of the Mail Editor to change the display size of the Web Page displayed in each Tabbed Window.

Photo Zoom Toolbar
Click on the Increase and Decrease size buttons (1) to move through the standard display sizes or select the size directly from the drop down list of sizes (2).
Selecting the Custom Size displays the Custom Zoom Window, as shown below:-

Custom Zoom Window
1. Display Size - type a new display size in the display size field as a percentage of the actual size of the Web Page.
2. Zoom size marker - alternatively drag the Zoom size marker left or right to increase or decrease the display size of the Web Page.
3. Zoom increment - type a new value in the Zoom increment field to set the incremental value used to increase or reduce the display size.
Select the Exit button to close the window, or select a standard size from the Zoom controls to return to one of the standard sizes provided.