Is there any constraints or advice about the size/shape of loading-logo.png? I can't see anything about it in the manual.
If you mean the main loading logo the appears similar to the construct 2 splash screen, you can go as wide and tall as your projects viewport, and as little as you want.
The 64x64 loading-logo.png in the icons folder is the default used size, but i don't think there are any limitations on it.