A "Statue" (f) tends to be at least life-sized and usually depicts a person or animal. A smaller one (e.g. foot-high) could be a "Statuette" (f) (but the word is rarely used).

Originally and in exact technical terms, a "Skulptur" (f) is made by taking things away (carving stone or wood) and a "Plastik" (f) is made by building the thing (e.g. from clay, or casting in bronze), but even the experts sometimes use them interchangeably. Personally, I'd call a thing made from stone or wood "Skulptur", and an artsy arrangement of some tin cans either "Plastik" or "Skulptur".

(By the way, note that "die Plastik" = the sculpture vs. "das Plastik" = the plastic (synthetic material).)