"Длина улицы — около двух километров."
Translation:The length of the street is about two kilometers.
I'm curious about this, too. Maybe because двух километра is already technically plural - even though as a sort of 'dual plural,' it takes the genitive singular form - introducing около kicks the nominal declension into genitive plural? I could be completely off base, or not explaining it well, but that's the only thing that makes sense to me.
Yes, but it's not just около.
The rule about using genitive singular with numbers applies only after the forms два/две, три, четыре (it's safer not to write numerals like '2' when talking about this!) - so два паспорта, две девушки, три окна, etc.
BUT: if the number itself needs to be in a different case (because of a preposition, or its function in the sentence), then the noun will be plural, in the same case as the number. Около requires genitive, so we use genitive двух, and genitive plural километров.
More examples: Я помогал двум студентам. I was helping two students. помогать takes dative, so we use dative двум, and студентам is dative plural.
Мы были в трёх городах. We lived in three cities. в + prepositional expresses location, so we use prepositional трёх, and prepositional plural городах.