djwebb2021 wrote:
Tá an bosca mór is fine.
Modern Grammars don't have detailed rules except for the rather simplified one: Use tá with adjectives!
According to them this would be correct.
But the examples given there are strikingly always only adjectives of a more temporary nature (te, tinn, breoite, etc.), never "mór", "beag" or the like.
That is so probably for a good reason: Adjectives of a more permanent quality are rarely used so.
A native speaker would rather never say: "Tá an bosca mór."
At least, I've rarely ever seen such sentences written. (And there should be millions of them.)
Of course, no one says "Is mór é an bosca", either. That is obsolete.
In fact, native speakers don't say "The box is big" at all.
Instead, they prefer "It is a big box": "Is mór an bosca é", "Is bosca mór é", etc.
Adjectives like "mór" are so rather always attributive adjectives ("big box"), they aren't used predicatively.
Some older books* have rules that confirm this in more detail.
I put this here:
Prädikative Adjektive mit bí (in German, of course

).
*) Unfortunately, I don't remember which, G. Ó Nualláin, C. Ó Cadhlaigh? Níl a fhios agam.