Population of Washington congressional districts/WA Secretary of State map
Congressional reapportionment can be hard to conceptualize unless you can picture the area in your mind: As mobile blobs, some overstuffed and others too thin. In the case of congressional districts, all are supposed to come close to a specific target, and when a new one has to be created out of the existing ones, the population for the new will have to come cheifly from the fat portions of the existing.
As you can see here, in this useful map from the Secretary of State's office. (You can get a better, clearer look via the link in the cutline.) The fattest (in population) congressional district is District 8, the area east and southeast of Seattle; by itself, that's approaching a fourth of the population for a whole new district.
Looking at the map, you can imagine slicing off maybe 135,000 people from the south of district 8, about 115,000 from District 3 to its south, and another 50,000 or so from District 9 to the west of them, to form a contiguous area ... that would still be less than half of what you need for a new congressional district.
So it's going to be more complicated than that, even before we start considering the question of where the current members of Congress live (and don't imagine that won't come up for discussion).
Sparks some thoughts, though.