Start with the letter names - E-F-G-A-B. That's going to be a 5th - of some sort.
Drop the E down a semitone, the interval's an augmented fifth. If, instead, the B goes up a semitone, that's also an augmented fifth. So, by dropping E to E♭, and taking B to B♯, it's a double augmented fifth interval.
Which sounds exactly like a far more common major 6th.
I would say Eb - B# is a doubly augmented fifth.
The letter name distance says it's a fifth: E - F - G - A - B, five different letter names (staff positions).
You probably already knew this, but just for future readers: what you call the interval depends on what you call the notes. As you can see from ...