Not in 1/3rd really, but I sculpt plenty of larger bits in wet clay for work. Long as you keep it wet its pretty easy to avoid cracking. Do you intend to fire it? if so, I'm definitely in no place to advise you...
but assuming you're either not going to be keeping it, or intend on moulding the sculpt after you're done, build up on top of a form if you've got one (an old bust or something makes a nice starting point- i often knock the face of old lifecasts to give me a basic shape and build up on top of that), failing that, buy a sculpting support from a sculptors shoppe or make one yourself (google it - loads of designs out there - simpler the better).
While you're roughing in, keep the clay moist and make sure you bed everything down to your support (or it'll fall apart like). Use tools more than your fingers to work the clay (helps stop it drying out) and always give it a spray of water and wrap it up when you leave it (i use cling film), making sure to come back and check that its still wet if you're leaving it for any extended period.
As you get your forms roughed in and you're ready for detailing, you can let the clay dry a little and this will help it take detail. Use water sparingly at this point - you want it moist enough so as not to crack, but not so moist that you're just squashing everything.
You could go to 1/3rd with lbt tho - I sculpt plenty big stuff with that and, as long as you're not too frugal with your armature packing, it shouldnt prove too expensive.
WED clay is generally quite nice to use btw.
not sure if thats helpful, but i'm bored at work and the typing has kept me awake!
robb
)
