I really like the name
Mariana. It's delicate and strong at the same time, somehow. Very beautiful.
I know a
Mariana and nobody has ever tried to shorten her name as far as I'm aware. I wouldn't be overly concerned about
Mariana getting chopped up into
Mare or
Mary because
Mariana just rolls so nicely off the tongue. If anything I might encourage a different nickname, like "
Yana" or
Mina or something.