Is this 70km crack in an ice shelf of Antarctica remarkable, or a regular occurrence?
Is an iceberg the size of Delaware [6,451 square kilometers] actually remarkable, or something that just happens from time-to-time?

In March 2000 iceberg B-15 formed which was 11,000 sq. km. and 295 km long.
In October 1998 iceberg A-38 formed which was 6,900 sq. km.
A Delaware size iceberg would be the 3rd largest in the past 20 years.
The 1990 article The calving and drift of iceberg B-9 in the Ross Sea, Antarctica Antarctic Science Vol. 2 , pages 243-257 says:

The largest iceberg in recent times
(95 km x 95 km) calved from the Larsen Ice Shelf in early 1986

So a 9000 sq. km. iceberg broke off in 1986.
Contribution of giant icebergs to the Southern Ocean freshwater flux says:

an iceberg of 10,000 km2 in area broke from the Amery ice shelf in 1964

According to Antarctica shed a 208-mile-long berg in 1956:

A record iceberg seen in Antarctic
Little America V, Antarctica, Nov. 17- The U.S.S. Glacier, the Navy's most powerful icebreaker, has sighted an iceberg more than twice the size of Connecticut.
The berg was sighted by the Glacier early this week about 150 miles west of Scott Island. The ship reported it was 60 miles wide and 208 miles long- or more than 12,000 square miles, as against Connecticut's 5,009.

So 12,000 sq. mi. or 31,000 sq. km. is the largest known iceberg.
Both of the two previous references also mention a 1927 Antarctic iceberg that was 170 km long, but area is not given.  American Practical Navigator, says the 1927 iceberg was 100 mile long and 100 miles wide, so 25,000 sq. km.
In summary, an iceberg the size of that contemplated in the OP can be expected on the once every 5-20 years time scale.
update 7/12/2017:
according to the MIDAS project:

The calving occurred sometime between Monday 10th July and Wednesday 12th July 2017, when a 5,800 square km section of Larsen C finally broke away. The iceberg, which is likely to be named A68