Hi @Lindsay - thanks for your question! When you configure the height of cards in a site/page layout you are specifying the absolute pixel value of the height. As you describe, this means 750px may take up most of the screen on a mobile device or appear somewhat small on non-scaled high-resolution screens.
We could consider an update to these to allow users to specify relative units (such as percentages or viewport height). If you're interested, please log an "Idea" in Esri Community or submit a formal enhancement ticket through Esri Support Services.
In the meantime, you might consider displaying these ExB applications via the Gallery Card and allowing users to open them within ArcGIS Hub on our full-screen content views. This will automatically display the application to the full height and width of the screen.
Hope this helps!