Learn Finnish: Minkä kokoinen epäilty on?

The adjective ending -kokoinen (sized) requires the word modifying it to be in the genitive case. Since we are asking "what size", the question word mikä (what) takes the genitive form minkä.

Why are minkä and kokoinen written as two separate words?

When the genitive modifier is a pronoun (like minkä, sen, or tämän), it is written as a separate word from the adjective (e.g., minkä kokoinen). When the modifier is a short adjective or noun, it is usually combined into a single compound word (like samankokoinen).

The word epäilty looks like a verb form. How does it mean "suspect"?

Epäilty is the passive past participle of the verb epäillä (to suspect). In Finnish, it's very common to use participles directly as nouns. So, the form that literally means "the suspected one" is simply used as the noun "suspect".

Why does the verb on come at the very end of the sentence?

In questions starting with a question phrase (like minkä kokoinen), the standard word order is Question Phrase + Subject + Verb. Here, epäilty (the suspect) is the subject, so the verb on (is) naturally falls at the end.