First, make sure your app is set to dynamically resize its width and height (see
Format > Layout options...) and that the other objects within your app (e.g. tabs, menus, forms, reports, etc.) are all set to resize in the manner you want. You can test this by running your app with the
Test App button in the design window.
Second, you can change the styling of the embedded iframe element to make it resize within its parent element.
Here is a simple example:
For more info on working with responsive embeds, you may find the following article helpful: