Set the icon size in the corner to whatever you want then copy and paste it in. I'm pretty if you copy and paste an icon bigger than 32x32 into BYOND it automatically adjusts the icon size. BYOND can't edit icons bigger than 32x32, though.
Make sure the "use size from file" checkbox is checked and when you import an image into a blank icon file, it will automatically adjust the icon size.
Set the icon size in the corner to whatever you want then copy and paste it in. I'm pretty if you copy and paste an icon bigger than 32x32 into BYOND it automatically adjusts the icon size. BYOND can't edit icons bigger than 32x32, though.
They said they may add that ability in the future, though.
You should have said, "How do I use icons bigger than 32x32?" You already have the icon, so it's no use asking us how to make them.
Anyways, what you do is simply create an obj set it's icon and icon_state, then change it's pixel_x or pixel_y and add it to the mob's overlays var.