Translation:The loaf of bread is big. It is a big bread.
I don't think "bread" is a countable noun in English. Maybe the answer should be something like "a loaf of bread" or something.
"Brot" can be used as countable and uncountable because when used as countable noun, you mean a loaf or a slice. "Ich esse Brot." (general) and "Ich esse ein Brot" (particular situation) are both grammatical.
Walty is right. It may be correct in German, but the English translation is wrong. Bread is uncountable and needs a quantifying phrase. It should be, "It is a large loaf of bread.
Just to make it clear: when "groß/klein" goes after the "ist", it's totally independent of gender and case?
There are two ways to use adjectives, predicate adjectives and attributive adjectives.
Predicate adjectives are the object of the verb sein (ist, bist, etc) or werden. Predicate adjectives do not show declension. Thus: Das Haus ist groß.
Attributive adjectives are positioned before the noun they modify and show declension based on gender, number, case, and a preceding article. Thus: Es ist ein großes Haus.
In both cases, the adjective can be after the verb ist, so that alone is not sufficient to indicate whether the adjective needs to show declension or not.