From what I've read, it should be at least one cubic yard for compost to heat up properly. Our compost pile is a wood frame with chicken wire stretched on it. DH basicly made 4 3ft x 3ft frames out of the scrap wood, secured a square of chicken wire to one of them (this will become a "door"), dug a hole in the ground (3ft x 3 ft and about 1 ft deep (less, actually, b/c he got tired
), then put 4 posts in the corners (used old fencing posts that were left by the previous owner), put hinges on one of the posts and the "door" frame. The other three frames he just wired to the posts and wrapped chicken wire around three sides. That's pretty much it. We cover it with old vinyl tablecloth and weight it with stones in fall when we think that it's getting too much rain on it. But you don't want to block ALL water access to the pile... It shout be damp/moist, not soggy.
One bin like this got us through the summer (we just moved to this house this summer). Then, DH made another bin like this, using one of the sides of the old one as a common wall-- this one for the leaves. Both are full to the top now, and will sit though winter. We'll decide next summer if we need to put the third one in to make a rotation easier.
I wanted to add that even though this is totally make-shift arrangement, and we did not use a single new thing to build it, it looks very neat.