Most stainless comes in a 36" width, at least the stuff we use at work here, with a 48" width being a special order.
And if it were me, I would not get "annealed" stainless, as it will dent much easier, and will "dimple" where you put the screws to mount it.
Also, it's only 18 thousandths thick, which is basically "shim stock" in my eyes.
If you are local to them, go there and talk to someone, and see if you can't come up with some at least "1/4 hard" stainless, of at least 40 thousandths thickness.......... .050 would be better.
Tell them what you are trying to do, and that you don't want it wrinkled as " a sack full of buttholes".
And the material should come in a 10 or 12 ft. sheet, so you can get a seamless backsplash.
Secure it to the wall picking up your existing studs, ( visible), with a "prime coated" deck screw or the like, ( or ones made for pressure treated lumber, they are gray), of at least an 1 1/4 length, ( 1/2 of which will be drywall thickness most likely)..........
I'm guessing you want it to be as smooth as possible, yes?
Look at the fasteners I mentioned above, and see if you want to countersink the holes that big, where the head will be flush.
Countersinking a piece of metal that thin will leave an ugly hole, so ya' may have to go with "pan head" or "truss head" type screws..........These last two options will be a plated, ( shiny) , screw.
If you go with 2 pieces of metal, make the seam fall on one of the studs, and have a strip of metal cut to cover the seam, and to hold the screws to secure the end of the two pieces.
Those are just my ideas............