Your 'default' thumbnails settings can be set here:
From there, you can also add a Thumbnail Size "slider" to any toolbar from within Customize->Command->View category, or you can create buttons and/or hotkeys to make use of commands like:
Show THUMBNAILSIZE=
Show THUMBNAILSIZE=reset
Note: that the "slider" will be hidden on any toolbar unless you're actually viewing the lister in thumbnails mode, but it will "appear" as soon as you enter thumbnails view mode. This is a little different than any "buttons" that you create on a toolbar using the sort of commands above, which will "remain" hidden unless you put them in a menu or "three-button" button, like the button definition below:
<?xml version="1.0"?>
<button display="both" icon_size="large" label_pos="right" type="three_button">
<label>Thumbs</label>
<tip>Thumbs</tip>
<icon1>#slider</icon1>
<button display="both" icon_size="large" label_pos="right">
<label>Thumbnails=128</label>
<tip>Thumbnails=128</tip>
<icon1>#slider</icon1>
<function type="normal">
<instruction>Show THUMBNAILSIZE=128</instruction>
</function>
</button>
<button display="both" icon_size="large" label_pos="right">
<label>Thumbnails=Reset</label>
<tip>Thumbnails=Reset</tip>
<icon1>#slider</icon1>
<function type="normal">
<instruction>Show THUMBNAILSIZE=Reset</instruction>
</function>
</button>
</button>
This button will set the current thumbnail size to 128 by clicking the left mouse-button, and then reset the thumbnail size to whatever your default is by clicking the right mouse-button. This is just an example... and unfortunately you have to use different mouse clicks or hotkeys to go back and forth - as it doesn't appear Opus supports "toggling" between two values for the 'Show THUMBNAILSIZE' function in a single command. Probably wprth a feature request...