How can I tell Automator to display an image with Preview but actual size?
Without seeing your actual Automator workflow, the only thing I can think of off the top of my head is to add a Run AppleScript action, after the action that opens the file in Preview, with the following example AppleScript code:
tell application &quot;Preview&quot; to activate
tell application &quot;System Events&quot;
    tell application process &quot;Preview&quot;
        if value of ¬
            attribute &quot;AXMenuItemMarkChar&quot; of ¬
            menu item &quot;Actual Size&quot; of ¬
            menu 1 of ¬
            menu bar item &quot;View&quot; of ¬
            menu bar 1 is missing value then
            
            click menu item &quot;Actual Size&quot; of ¬
                menu 1 of ¬
                menu bar item &quot;View&quot; of ¬
                menu bar 1
        end if
    end tell
end tell

This is similar to manually pressing ⌘0 while in Preview, which activates the Actual Size menu item on its View menu, if it is not already showing its actual size.
This also assumes the default preference of Define 100% scale as: 1 image pixel equals 1 screen pixel is in play.

 Note: Using the example AppleScript code may require adjusting settings under: System Preferences &gt; Security &amp; Privacy &gt; Privacy