この島の面積は約70平方キロメートルですって英語でなんて言うの？

Square kilometres refers to the amount of space that an object or land takes up. So when you refer to something that is 70 square kilometres it means that when you multiply one length or the island by another it will equate to 70 kilometres.

An example is:

A rectangle with a length 5km and 4 km has an AREA of 20 square kilometres. This is because we multiply 5 and 4 together.

To tell the size of an island, you can state it as "The island's about 70 square kilometers."

"The island was like 70 square km." this is an informal way to say that the island's area was about 70 square km. It is unnecessary to mention "area" because its already indicated by saying "square km". "Like" can replace "about" to appear more relaxed while still suggesting that the measurement is an estimate.

The total area of something is the amount of space measured in square metres or larger or smaller measurements depending on the context.

When you want to explain to someone that an island you visited recently is quite large, and you want give them the approximate or exact size of it; then you can say it in the following ways:
-I visited a large island recently. It is about 70 square kilometers.
-The area of the island is about 70 square kilometers.