Learn Finnish: Vainajan omaisuus on suuri. - The deceased's property is large.

To ask "Whose property is large?", you would replace the genitive noun vainajan (the deceased's) with the genitive question word kenen (whose). The question simply becomes: Kenen omaisuus on suuri?

The word omaisuus looks familiar. Is it related to the word oma (own)?

Yes! The word oma means "own" (as in "my own"). By adding the suffix -isuus, which turns words into abstract nouns, it becomes omaisuus, meaning "property", "assets", or "wealth".

Could I use iso instead of suuri to say the property is big?

You could, and people would understand you, but suuri is the better choice here. In Finnish, iso is typically used for physical size (like a big house), while suuri is preferred for abstract concepts, amounts, and wealth.

Why doesn't the word omaisuus have a possessive suffix here, like omaisuutensa?

When the owner is specifically named in the genitive case right before the possessed thing (like vainajan), the possessive suffix is not strictly required. It is perfectly natural to just say vainajan omaisuus.