Hey everyone!
I was able to figure out the issue. Essentially I was thinking about it wrong, instead of trying to find the right sized image I needed to scale the image on the html5 page. After some experimentation and research. I found that adding: background-size: contain; to .canvas-app-canvas allows the image to scale with the window.
I copy / pasted the built in css file and added the code to this part to create this:
.canvas-app-canvas {
background-repeat:no-repeat;
background-position: center center;
{{#DEFOLD_SPLASH_IMAGE}}
background-image: url("{{DEFOLD_SPLASH_IMAGE}}");
{{/DEFOLD_SPLASH_IMAGE}}
{{^DEFOLD_SPLASH_IMAGE}}
background-size: 70%;
background-image: url(...); /*keep the long URL featured in the built-in css, not featured in this forum post to save space*/
{{/DEFOLD_SPLASH_IMAGE}}
background-size: contain;
}
Hope this helps anyone who was dealing with the same issue!