Learn Icelandic: Þessi húsbíll er mjög plássmikill.

The suffix -mikill (meaning "great" or "much") attaches to nouns to describe an abundance of that thing. Here, pláss (space or room) plus -mikill creates an adjective meaning "spacious" or "roomy". The opposite would be plásslítill (cramped or short on space), using the suffix -lítill.

Why does plássmikill end in -ill in this sentence?

It must agree in gender, number, and case with the noun it describes, which is húsbíll (masculine, singular, nominative). If we were describing a feminine noun like íbúð (apartment), it would drop the second L to become plássmikil. For a neuter noun like hús (house), it would become plássmikið.

Is húsbíll a literal translation of "house-car", and how do we know its gender?

Yes, húsbíll is literally "house-car" and is the standard word for a campervan. In Icelandic compound nouns, the grammatical gender is always determined by the final word in the compound. Since bíll is masculine, húsbíll is masculine, which dictates the masculine "this" (Þessi) and the masculine adjective (plássmikill).

Is it more natural to use plássmikill instead of saying the van "has a lot of space" (hefur mikið pláss)?