I just commented on this over here , but for better visibility will post it here as well:
var s = Vector2(2.0, 2.0)
cardn[cardcount].expand = true
cardn[cardcount].rect_min_size = cardn[cardcount].rect_size * s
This will only work of course, if
rect_size is set to the size of the texture. Which will happen automatically when you add a texture in the inspector, but has to be done manually if you're doing that from within GDscript. I assume this won't be an issue, given that your designing a card game and I expect all cards to have a uniform size.