Learn Croatian: Njegov portfelj je jako velik. - His portfolio is very large.

Velik is the indefinite form of the adjective. In Croatian, when an adjective is used as a predicate (meaning it comes after the verb to describe the subject, like in this sentence), it usually takes the indefinite form. You would use the definite form veliki if it were directly attached to the noun, as in njegov veliki portfelj (his large portfolio).

Why does the clitic "je" come after the two words "Njegov portfelj" instead of right after "Njegov"?

Clitics belong in the second position of a sentence. However, Njegov portfelj (His portfolio) is a single noun phrase. While splitting it with a clitic (Njegov je portfelj...) is grammatically possible, it sounds overly poetic or formal. Treating the entire noun phrase as the first grammatical block is the standard, natural word order.

Because "portfelj" ends in "lj", does it decline differently than standard masculine nouns?

Yes. Because lj is a soft (palatal) consonant, portfelj follows the soft masculine declension. This means it takes the ending -em in the instrumental case (s portfeljem) rather than the hard -om, and its vocative ending is -u (portfelju) instead of -e.

Could I use "vrlo" or "veoma" instead of "jako" here?

Absolutely. Jako, vrlo, and veoma all mean 'very'. Jako is the most common choice in everyday spoken Croatian. Vrlo and veoma can sound slightly more formal or literary, but they are all perfectly correct and interchangeable in this context.