A wild shaping druid will often change size category -- for example, a human druid (medium size category) might change shape into a dog (small size category). The alternate form special ability on which wild shape in base simply says that the druid "gains the size of its new form" without specifying what that means. I'm trying to figure out exactly what all changes as a result.
Is the Size Increases table from the section on improving monsters relevant? I suspect not, but I'm not sure.
Does the druid get a bonus/penalty to their Hide skill, as per the Hide skill description?
Does the druid gain a size bonus/penalty to attack, AC, and grapple per the Big and Little Creatures in Combat table?
Does the druid's reach change?
The rules for this just seem to be scattered hither and yon.