There does not seem to be one.

Control + mouse scroll will adjust the font size, but not the overall scale of the display. This is especially annoying when at the default zoom level some of the text falls outside the the e-reader window:

Cumbersome, but at least the text will become readable: CSS to the rescue.

• Right click in the reader window
• Select ‘Preferences’
• Select ‘Styles’
• Add a new scaled body style in the text box
• Click OK
• Close the ‘Preferences’ menu
• Start reading

The CSS properties added are:

body {
  transform-origin: top left;
  transform: scale(1.0);
}

Why this is not available via the GUI… ?

P.S. The new style is stored in .config/calibre/viewer-webengine.json