Learn Finnish: Onko hän niin voimakas kuin ihmiset väittävät?

The adjective voimakas is created by adding the suffix -kas to the noun voima (power or strength). This suffix indicates that someone or something possesses an abundance of that quality, translating perfectly here to "powerful".

If the subject was plural (e.g., "Are they as powerful?"), how would the -kas ending change?

Adjectives ending in -kas have a stem that changes to -kkaa- before adding plural or most case endings. Because plural predicative adjectives take the partitive case, the word would become voimakkaita (Ovatko he niin voimakkaita...).

Could I say yhtä voimakas kuin instead of niin voimakas kuin?

You could, but the nuance shifts slightly. Yhtä ... kuin strictly means an equal comparison ("equally as powerful as"). Niin ... kuin is very common in questions and negatives, translating more naturally here to "Is he that powerful, like people claim?"

The verb väittävät stands alone here. How does väittää usually connect to the thing being claimed?

In this comparative sentence, the object is implied by the structure. Normally, when you state what someone is claiming, väittää is followed by an että (that) clause. If you use a direct noun object with it instead, it requires the partitive case.