Since Ingage supports the iPad Pro, the assets you choose should be big enough to display on a Retina 12.9" screen. In resolution terms, for a full-screen image you are looking at a minimum of 2732 x 2048 at 264 pixels per inch (ppi).
If your assets are significantly larger than that, you may want to resize them (down) to them get closer to that spec. Doing so will reduce your overall story size, which makes it download faster and take up less space on your customer's device. One caveat to keep in mind, however, is that if you plan to zoom in on an image within the Ingage app, you'll want to keep it larger to avoid any loss of quality.
If your original images are smaller than the above recommended resolution for iPad Pro, we recommend you do not make them larger. Doing so will not improve their quality in the app. If your images are below this spec and do not appear sharp enough when testing in the app, we suggest finding larger images to use.
Most video editing programs allow you to export video based on the device it is intended for. Choosing "iPad" will ensure that it is high-quality enough for iPad Pro screens and for web browsers (for those customers viewing your story in the web view).