If you want to open a modal dialog with event details after event resizing is complete, the best way is to open the modal dialog using AfterRenderJavaScript. This event is fired on the client side during the initial page load and after every callback.
In the EventResize server-side event handler, you can call Update() with custom data. This data object will be available in AfterRenderJavaScript. See here:
http://doc.daypilot.org/scheduler/callback-update/
If you try to open the modal dialog using EventResizeJavaScript the server-side EventResize is not finished yet in most cases and you will get an old version of the event (before the update).