"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"?
Why is there no accusative suffix?
Because accusative is only used when something is being acted on - if it just "is" it stays nominative
Because the verb "to be" is implied.
There is no direct object/accusative in sentences with the verb "to be". "Vannak", 3rd person plural of "to be" is implied here.