The noun is actually singular when used this way, but it's spelled the same, so it's confusing. It means a singular type of specific people, as an entire group, such as a nationality or ethnicity. “Norwegians are a tall people,” for example, or “Americans are a proud people.” You see a similar construction (a definite article with singuar “people”) in “The people of this land.”
It's an uncommon construction in English, but does get used often enough that my first assumption when seeing „Wysocy ludzie” was ”A tall people”, not ”Tall people.”
Thank you! These and the replies below by mihxal, immery, and Emwue have been very instructive. Having separate words (or as below, a different word order) to mean approximately what ”a people“ means in English is quite sensible of Polish, and much less ambiguous than how English does it. :) Dziękuje wam!
Well yes, it's plural. The questions is can „Wysocy ludzie” in Polish ever mean an entire ethnicity is tall, or if it can only mean a bunch of people are tall. Since „Dobrzy ludzie” can mean “The good people”, which in English is sometimes the same as ”A good people”, it seems like „Wysocy ludzie” could/should also mean ”The tall people” or ”A tall people”.
Only if you change the order and put noun before adjective:
(Ci) Norwegowie to wysocy ludzie = (These particular) Norwegians are tall people
Norwegowie to ludzie wysocy = (all) Norwegians are tall people = Norwegians are a tall people
Other than that, as immery, mixal and Vengir already pointed you can use words like "lud", "naród" or "ludność".