The Shepherd Planar class declares:
Wild form (Sur): Your druid levels are superimposed with your planar shepherd class levels to determine the number of daily uses, the maximum HD and size (but not the type of creature), as well as the duration of your wild form technique. .
The Druid class defines the animal sizes that you can turn into druid-related levels: small and medium on the 5th level, large on the 8th level, tiny on the 11th and huge on the 15th.
Could a Level 7 druid, a first level planar shepherd, turn into a big animal?