ANY mophead hydrangea variety will work in a PNW zone 7. Mature heights vary but most will get to be of significant size........about 6' tall and as wide (if not wider). There are smaller or even dwarf forms - Pia, Pistachio, any of the City Line series - but they tend not to have the same flowering impact of a full sized hydrangea shrub.

Is there a flower color preference? Hard to go wrong with the standby Nikko Blue.