six-footer vs someone

six-footer and someone both are nouns.

Word	Noun	Adjective	Verb	Adverb
six-footer	Yes	No	No	No
someone	Yes	No	No	No

As nouns, someone is a hypernym of six-footer; that is, someone is a word with a broader meaning than six-footer:

six-footer: a person who is at least six feet tall
someone: a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential

Other hypernyms of six-footer include individual, mortal, person, somebody, soul.

six-footer (noun)	someone (noun)
a person who is at least six feet tall	a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential