Why isn't it correct to say "Ez EGY nagy szálloda"?
It is. You can say it both ways.
But I can not always leave the indefinite article, can't I? In which situations is only one possibility right? (Exect of the meaning of egy=one, that's clear to me.)
You can't omit it when there is no adjective before the noun.
Would not it be correct "Ez magas szálloda." too?
'magas' = 'tall'