uhm... Big and Weird-shaped aren't very helpful.
Does it have strings? The string Bass is the biggest of the stringed instruments. However: If it has 4 strings, it is a String Bass. If it has 6 strings, it is a Violone.
Does it have pipes? Many orchestras have pipe organs built into their Halls, which play as soloists or part of the orchestra for certain pieces. On the other hand, if may appear to have pipes, but also have a hammer-like beater: this is the Orchestra Bells.
Does it just have metal tubing, and get really big at one end? It's probably a tuba, if the tubing bent into an oblong shape, but if the tubing is bent in an "o" shape, it is properly called a Sousaphone (or Helikon.)
Is it partially wood? (Usually, the wooden parts are a deep red, and the metal parts and keys are silver.) It might be a Contrabassoon.
Is its weird shape because it is round like a drum, but pictured at an angle, and it has a large metal structure holding it up? It's a Bass Drum. (The largest bass drum used by a marching band is owned by the Purdue "All Americans" and is about 8' diameter and about 4" wide: it rides on a carriage made from a Ford Model "T" back axel and wheelbase: there's an article on it "Purdue Big Bass Drum" on Wikipedia.)