Can a character with Wild Shape or casting Form of the Dragon/Monstrous Physique/Elemental Shape/etc. shift into any form (meeting the size requisites) that the player knows of, or do they need knowledge of the for form to be taken?
None of the spells you've listed mention a familiarity requirement, so by default there is none.
At 4th level, a druid gains the ability to turn herself into any small or Medium animal and back again once per day. Her options for new forms include all creatures with the animal type. This ability functions like the beast shape I spell, except as noted here. The effect lasts for 1 hour per druid level, or until she changes back. Changing form (to animal or back) is a standard action and doesn’t provoke an attack of opportunity. The form chosen must be that of an animal the druid is familiar with. ... As a druid gains levels, this ability allows the druid to take on the form of larger and smaller animals, elementals, and plants.
So for animals, Wild Shape requires familiarity. The wording is not entirely clear on whether this requirement extends to plant and elemental forms.
For elemental forms the ambiguity doesn't matter much, because a druid can easily use Summon Nature's Ally to acquire familiarity before they get the appropriate Elemental Wild Shape ability. So plants are the major area of uncertainty.