I believe that the the preview tool is meant to be a way to quickly look at the data and styling for the admin, it's not intended for customization as far I as know.
The documentation on the layer preview page provided on the geoserver website doesn't provide any methods on how to change the layer preview page width and height values as far as I could find.
I've come across this discussion which alludes to a possibility of achieving this with a modification to the java code and or in the URL at specific parameters.
On a separate note, utilizing the
GetMAP method certainly allows for the specifying of the width and height output, which you could read more about here