The first problem is do you have a large enough oven to cook this
loaf in? My normal recipe for challah uses 2 pounds 11 ounces of
flour and creates 2 medium/large loafs. Each takes up a full half
sheet pan. So for the recipe you're talking about you will need an
oven that can handle a full sheet pan.
Then you have a couple options to make it fit on the sheet pan.
One bakery I worked at, braided a "U" to fill the sheet pan. You can
do this or other designs to fill the pan, but I do not think it will
give you the presentation you want for a wedding.
Another option (which will require a bit more work & skill) is to
create multiple braids and place them on top of each other. This will
give the loaf much more height and a more complex look. Think of
doing 2 or 3 braids, like a pyramid. The largest braid on the bottom,
the next one up smaller, etc. You can do the same with complexity as
well. Example: 6 strand braid on bottom with a 3 strand braid on
top. While I have not made one this way, I've seen them and it can
make a very beautiful loaf.
Any challah recipe should work.
Steven