Hi @Dentonthebear,
Sorry for late reply. We have no restriction on size when you choose custom icon. We resize an provided image to 200x200 pixels, after resizing if its size is less than 100KB, it is used as custom icon otherwise not.
So, a Custom icon with 200X200 pixel and less than 100KB will always be accepted.
Also, favicon support is coming soon and it will save your time from adding custom icon for every website.
Thanks.