Every thing indicates—the smallest does, and the largest does,
We think that this section points back to Section 3 and the discussion of the "well-joined scheme," the grand design of the world. We can never see the whole scheme, but "every thing indicates" what it looks like. Small things tell us just as much about the world as large things do.
You can see why Whitman is a favorite of environmentalists and biologists everywhere.
A necessary film envelops all, and envelops the Soul for a proper time.
He has been saying all along that all things are connected, but now he gets a little more specific (with emphasis on "a little").
Everything, he says, is "enveloped" or contained by a "necessary film." Note that "film" doesn't mean "movie" – those weren't invented yet. It means something like a thin, invisible substance that coats everything, the way that wax coats leaves. Bubbles, for example, are composed of a kind of "film" (as are the soap scum that coats your bathroom walls).
But Whitman's uses "film" in a positive sense. The film also coats the "Soul" – as long as we're alive.