Bach's Matthew Passion is pretty amazing. It's also pretty long and, by Baroque standards, rather grand in terms of the ensemble--two choirs, an orchestra, an organ or two. It's around 3 hours long, but it incorporates a pretty varied array of song styles, from the grandeur of the opening "Chorus" to the chromatic oddness of "Ach Golgotha." Get one of the recordings that's on period instruments, though, since Bach is pretty specific about what's playing what and several of the most important instruments in the passion (the two types of oboes he uses and the Viola de Gamba, particularly) have fallen out of use. Also, though you said you preferred larger-scale works, Bach's organ music is totally metal.
Stravinksy is pretty awesome, his Symphony of Psalms and Mass are less dissonant and percussive than his earlier, more famous compositions mentioned above, but they are really, really good and pretty musically interesting.
Dvorak has to be one of the most accessible classical composers ever. You probably know at least one movement of From the New World aka Symphony no 9 already. The rest is also good. Also in this category: Beethoven. The ninth symphony is a hell of a lot more than the "Ode to Joy." Driving back from Wild and Wonderful West Viriginia with my girlfriend listening to it has to be one of the better moments of my life.
Wagner's Tristan and Isolde is great if you don't mind music that feels like a bunch of really dissonant chords refusing to do what chords are supposed to do. If you prefer a melody, though, it's probably not what you're looking for. I can't decide if I love it or hate it, probably both.
Mozart's Requiem is really, really good, though not strictly speaking all Mozart.
I'm also really into Renaisance music, but that's not everyone bag since it's frequently so consonant as to strike many modern ears as boring.