This sentence on NHK's Easy News got me wondering:
It is shaped like a U and is about 1 km in diameter.
At this website
I found いっきろ
but shouldn't it be
or possibly even
As always in language learning, there is little point for a learner in trying to think about what the correct answer to this question should be; one just needs to know what native speakers do.
Here are three more examples from Jim Breen's wwwjdic:
It was less than one kilometer to the village post office.
There is a bridge about one kilometer up this river.
A gas station is one kilometer ahead.
(As always, the wwwjdic does not provide readings)