Here is a better way, in my experience, to set up TheBat! for zooming in & out, explained here separately for viewing and editing messages. This is for the classic viewer (I haven't tried the conversation viewer).

Why better?  What's below puts a (+) and (-) icon (actually magnifying glasses) in the Editor menu, and in Message Viewer text labels that are "Zoom in" and "Zoom out".  With this, if my keyboard tray is closed, I can zoom in/out with only the mouse, instead of using a Ctrl-Middle-Wheel which takes using two hands requires opening the keyboard tray.  I have a laptop and a docking station (with my laptop away from the desk). At the desk I am farther from my monitor (than when typing directly on the laptop), so in that case zooming is frequent and important both for reading and creating messages.

HOW FOR THE EDITOR:  Start a new message, right-click in the toolbar, and sel ect Customize. In the Left pane (which controls what is in the Right pane) select Standard. In the middle pane, open the AllActions item, then the MessageEditorForm item. Find the Zoom In and Zoom Items and individually move them to the right pane, using the Right arrow. They should be under the Options item there, but can be moved up.

HOW FOR THE VIEWER:  Open and existing message, right-click in the toolbar, and select Customize. In the left pane select Standard. In the middle pane, open the AllActions item, and move the Zoom-In and Zoom-Out text items items to the right pane. I moved mine up to between the forwarding items and the address book item.

FUNCTION:  Using Zoom In / Zoom Out in the Viewer is a little frustrating, because clicking one of those brings up a ZOOM POPUP, which is just a distraction in the way, and completely unnecessary because my mouse is right over the button.

JUST NOTICED:  When adding the zoom buttons for the Viewer, in the middle pane you can right-click on those zoom items and pick one of the (+) (-) magnifying glass icons fr om a (long) menu. In that case you get the icon instead of the text label.