A Medium humanoid druid that employs the supernatural ability wild shape to assume a Medium alternate form can, for example, consume a potion of enlarge person to gain that spell's benefits—like becoming Large and a +2 size bonus to Strength—while in that alternate form.

If that same (normally Medium but currently Large) humanoid (but having assumed an alternate form) druid then opts to end the wild shape effect while the enlarge person effect's duration continues, the druid resumes his normal original form except that the druid's Large as per the spell enlarge person until the enlarge person effect ends. (N.b. the spell enlarge person is dismissible.)

Were that same druid to then employ wild shape again to assume a Medium form again, he would assume that Medium form, the special ability wild shape setting his new size category overruling the ongoing enlarge person spell effect (q.v. One Effect Makes Another Irrelevant and—more tellingly—the omitted-from-the-SRD examples on PH 172).

So you know, this DM would likely rule that were the Medium humanoid druid to employ initially the supernatural ability wild shape to assume a Large or bigger alternate form, consuming the potion of enlarge person would have no effect. The enlarge person spell's description says, "Multiple magical effects that increase size do not stack" (PH 226), and the supernatural ability wild shape is, indeed, magical, therefore having the same constraints on its interaction with the spell enlarge person as any other magical effect.

However, this player wouldn't argue with a DM that ruled that because the supernatural ability wild shape sets a druid's alternate form to particular size that an enlarge person effect could work normally on that druid. That is, the supernatural ability wild shape doesn't change the druid's size but puts the druid's size at certain point and the enlarge person effect works from that point. This player may point out that this gives the druid an edge when employing size-changing magic—which is already pretty awesome—, but, since druids are involved, I don't think this ruling would even crack the top 10 list of Things I Worry About When Fighting Druids.

They are a statue of the beast form.

Petrification would not change their form, except to turn them into a stone statue of whatever beast form they were in. There is simply nothing in the definition of Petrified or the description of Wild Shape to indicate otherwise.

When reduced to 0 hit points, their statue appearance does not change, but you substitute back in the druid's usual stats.

The Petrified condition states:

A petrified creature is transformed, along with any nonmagical object it is wearing or carrying, into a solid inanimate substance (usually stone)

So the material of the now petrified beast form is solid and inanimate. However, conditions propogate through WIld Shape transformations. So when reduced to 0 hit points and Wild Shape ends, they are still petrified.

Now, when reduced to 0 hit points, Wild Shape states:

You automatically revert if you fall unconscious, drop to 0 hit points, or die.

So you revert back to being your usual self, as in, you are back to using your normal druid stats. But because your physical form has been petrified and is now solid and inanimate, the form of the stone would not change into your usual druid form until the petrification ends.

To give a needed caveat, this is, I think, the best reading of the rules for this interaction, but there is room to rule that the appearance of the statue is now that of the druid. And to be clear, the only difference between these two rulings appearance. Regardless of how you rule the appearance of the statue, the statue goes back to using the druid's usual stats when Wild Shape ends.