If you already have a document built and want to quickly set it up to scale up and down based on a browser’s width without creating any additional Responsive Layouts, read on:
First, set the document to scale in both the Width and Height Dimensions:
Next, add all your objects to a group, and set these properties on the group:
Below is a video walkthrough on how this works:
Also discussed here: Proportionally scaling a Tumult Hype document based on the browser width