We are talking about view in the Mail Panel (with the list of messages and viewing message bellow/right to the list)?
You can assign new custom keyboard shortcut (for an example - Alt+Z, but you can use a single-key shortcut too) for fast scale up mail message to 150.
Edit your keyboard config:
Add new keyboard shortcut in the [Mail Window] section
z alt = Focus page & Zoom to, 150
I've found that the custom mouse gesture will be more handy (e.g. GestureRight in [Mail Window] section). But the Opera Mail is just stump of the Opera 12, it doesn't have a mouse gestures support (unlike original Opera 12). For unknown reasons the mouse gestures (fantastic feature for opera mailer) was removed from Opera Mail.
Also you can use a custom keyboard shortcut (or custom toolbar buttons) to open Received, Sent and other mail folders with a predefined zoom setting. For an example:
Read mail,2 & Delay, 1000 & Focus page & Zoom to, 150 - open "Received" mail folder with zoom 150%
Read mail,5 & Delay, 1000 & Focus page & Zoom to, 150 - open "Sent" mail folder with zoom 150%
etc.