one site said it should be used as an apposition to a noun.[...]
Hello Manfy,
If the site really said this, I think you are right to pay it little attention, because I don't see how an adjectival phrase (
writ large) can be in apposition to a noun.
Maybe they were trying to say that
writ large usually follows the noun it refers to. But it doesn't have to at all. Here is an example from the British Corpus where it doesn't:
Here, truly, one can see writ large the truth of the Canal Commission's simple motto ββthe Land divided, the World unitedββ or applaud the brief laureateship of the poet Stafford who wrote, to celebrate the Canal's opening day in 1914 etc. The Pacific. Simon Winchester, London: Arrow Books Ltd, 1992.
Sometimes the phrase is used simply to mean 'written in large letters', as here, in another example from the British Corpus:
Wick, where the biggest shop is Woolworths and the young girls pass reeking of cigarette smoke, and CHRIST DIED FOR OUR SINS is writ large on the exterior of the Harbour Mission wall. Highland journey. Mairi Hedderwick, Edinburgh: Canongate Press plc, 1992.
I'm not sure that this use in the article in the Washington Post quoted in the OP is all that unusual. It is suggesting that Donald Trump was giving these three words, REFUGEES, MIGRANTS, and MUSLIMS, warning status - a flashing warning to indicate that the three categories of people should not be allowed to enter the United States.