There are essentially two ways to make a Juicebox gallery responsive (with its dimensions dynamically changing with the size of the user's browser window).
Scenario #1:
A Juicebox gallery will be responsive (and will dynamically scale with the size of the user's browser window) if the gallery's own dimensions and the dimensions of all parent containers are expressed as percentages. If there is a fixed value anywhere up the chain, then the gallery's size will become fixed (e.g. 100% x 100% x 800px = 800px).
Please note that when using percentage heights, you may need to implement the suggestion noted here.
Scenario #2:
You could use JavaScript to listen for a change in the size of the user's browser window and assign new dimensions to the Juicebox gallery if and when this happens.
An example of this can be found in the resizable galleries support section here.
Take a look at the source of this sample gallery in your browser to see how this might be achieved.
Both scenarios above apply equally to Juicebox-Lite and Juicebox-Pro.
With this in mind, it can be difficult to ensure that a Drupal web page conforms to Scenario #1 and also difficult to implement Scenario #2 within a Drupal environment.
The code you provided includes the following line:
<div class="juicebox-parent " style="width: 540px; height: 450px;">
It looks like your gallery's parent container has fixed pixel dimensions and, therefore, your gallery will ultimately have a fixed size, too (even if it has dimensions expressed as percentages).
I'm not sure where the dimensions for your 'juicebox-parent' container are coming from (the module or your Drupal theme).
(Please note that the module is an unofficial plugin which we did not write ourselves.)
If these values are coming from you current gallery dimensions, then try setting your galleryWidth to be 100% (and set your galleryHeight to be a fixed pixel value (such as '600px') as your Drupal page will likely support vertical scrolling in which case the gallery would not benefit from being responsive in the vertical dimension).
Otherwise, you might like to try a different Drupal theme which does not use containers of fixed dimensions (which will constrain your gallery).