We've known since 1977 that Leia knew she was carrying plans for something; if she didn't know before the film began, she certainly knew by the time she first runs into Vader:
Vader: Several transmissions were beamed to this ship by rebel spies.
I want to know what happened to the plans they sent you.
Leia: I don't know what you're talking about. I'm a member of the Imperial Senate on a diplomatic mission to Alderaan.
A New Hope (1977)
But a more specific answer comes from the new (2015) junior novelization of A New Hope; Leia knew she was carrying the Death Star plans, and is aware of rumours about a battle station, but doesn't connect the Death Star with that station:
All they needed to do was intercept a transmission about a new battle station the Empire was rumored to be building.
But the system had been swarming with Imperials. They'd caught on to Leia's bogus tale about her ship's breaking down, but the cover had helped her stall long enough to download the technical plans for something code-named "Death Star."
The Princess, the Scoundrel, and the Farm Boy Chapter 1
It's unclear whether or not Leia was aware of the "Planet-Killer" weapon that the Rogue One rebels keep banging on about, or whether she would have connected those rumours to the more general "battle station" rumours. Regardless, she eventually pieces it all together when she's first brought onto the Death Star:
"Death Star..." The words came out in a horrified whisper.
A second too late, Leia realized her grave mistake.
Vader turned on his heel. "Yes, the Death Star. The secret weapon you claim to know nothing about. The plans you claim you do not possess.
The Princess, the Scoundrel, and the Farm Boy Chapter 4