Why in a sentence "Jsme poslední muži" ("we are the last men") we have to use "THE" before "last men" but in a sentence "Jsme velcí muži"("we are big men") we do not have to use "THE".
Because 'Last" in English always has to have THE to make it unique. To point out it is THE last. With Big we would need to use the article if we wanted to say They are THE BIGGEST men.
The last is in reality a superlative.