Translation:These are large rivers.
"These rivers are big." would be "Ezek a folyók nagyok."
Can this also mean these big rivers?
No, that would be "ezek a nagy folyók".
These are large rivers.
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Could it also be "these rivers are big"?
There is no direct object/accusative in sentences with the verb "to be". "Vannak", 3rd person plural of "to be" is implied here.
Why is there no accusative suffix?
Because the verb "to be" is implied.
Because accusative is only used when something is being acted on - if it just "is" it stays nominative