Learn Finnish: Suurin peto oli todella vihainen dinosaurus. - The biggest predator was a really angry dinosaur.

Suurin is the superlative form of suuri, which is another very common word for "big" or "large". While iso becomes isoin (biggest), suuri is irregular. Its comparative is suurempi (bigger) and its superlative is suurin (biggest). Both suurin and isoin mean the exact same thing, though suurin can sometimes sound slightly more formal.

Why use todella here instead of tosi for "really"?

Both todella and tosi mean "really" or "truly", and both are completely correct here. Tosi is heavily used in everyday spoken Finnish (like tosi vihainen). Todella is technically an adverb form built from the root of tosi, and it sounds just a bit more emphatic or written, which fits a dramatic sentence about a dinosaur perfectly.

Why are both peto (predator) and dinosaurus in their basic dictionary forms (nominative case)?

This is a classic "A is B" sentence. When linking two singular, countable nouns with the verb olla (to be), both the subject (suurin peto) and the thing it is being described as (dinosaurus) stay in the basic nominative case. You do not need the partitive case here because a dinosaur is a whole, countable entity, not a mass noun or a substance.