I've never heard of a Giant Leghorn. That doesn't mean that someone, somewhere hasn't developed one. I don't really think there would be a reason to breed them, though. Leghorns lay very large eggs, and a lot of them. The larger the bird, the poorer the feed/conversion ratio. What that means is that you need to feed more food (ie. spend more $$$) feeding larger birds and you still get the same number and size of eggs.
I have no problem admitting I still don't know all the breeds I have... still learning
That's okay! We're all still learning!