If we were to take the idea of a "West Pacific" seriously I feel like it would be most meaningful to have it be defined by some major land obstacles that carve up the Pacific. In that case there are three possibilities:
1) Most expansive, it extends out among all those little islands until you get to the vast areas of open ocean. The North Pacific is rather empty. Hawaii sits way out by itself. Most of the islands are part of a big sweep all the way from China out towards New Zealand. All of that would be the West Pacific. Different from the South Pacific because this would be including Japan and the Philippines.
2) Define it vary narrowly as those bodies of water between the three great Asian archipelagos (Japanese, Philippines, and East Indies) and the continent.
3) With a certain stretch, defined the Indian Ocean as part of the Pacific and hence the West Pacific.
I feel like #2 is the most reasonable.