I've tried 32x32, 64x64, and I've tried to use Photoshop to edit another app's icon into my icon to keep its size but it never works with me. It seems that my icon is double-sized and I can't get the whole icon to be visible.
You can append @2x after image's base filename to mark it as a high resolution image.
This was helpful for me: https://www.electronjs.org/docs/api/native-image#high-resolution-image