One of the problems with SharePoint Apps is that you cannot SP.UI.showModelDialog() to show additional details to the end user. It is also a problem when you want to provide a richer UI than the very basic properties you can add to the ToolPart window.
Things you need to know
The App Part XML
In the project you will notice that the webpart elements.xml has a couple of changes.
Firstly the height is set to 0, If you know the default height use that, but 0 ensures that nothing is shown until the first resize has been performed. This can be useful to prevent things moving around in front of the user.
The editmode query string parameter has been added to the source url. This allows us to show a different dialog when we are in edit mode for the app part.
To read more about this click here, http://blog.thekid.me.uk/post/Dynamically-resizing-a-SharePoint-App-webpart-to-show-dialogs