One note on the impression from reading about pnw flies - I agree that most publications are prone to using the 2-1/0 range as common hook sizes, but this should not be thought of as limits on either end. Many people fish tiny flies with success, as well as huge flies.
I think the key difference is that in the PNW people move the fish to the fly whereas in the very cold winter fishing conditions I saw on Lake Ontario tribs the fly is moved to the fish, which are barely thawed in the icy water.
In "winter" fishing, flies of #2 to #2/0 or even bigger are well suited to the often glacially tinted rivers of the pacific northwest. Most flyfishing is done in late winter and the seasons are infinitely milder than other parts of the country at that lattitude. There is never split shot or slinkies or weight added so the fly operates on it's own weight and swimming ability is really key as opposed to creating the impression of a small vulnerable object like a nymph which was the more common presentation I saw on the Salmon River in Pulaski NY (made 3 trips there). But bigger and smaller winter flies still work, but are not the most common.
In summer, the use of too small a fly will result in literally hundreds of smolts. You don't want to go too small when the river is bursting with all kinds of juvenile species. I prefer to go with #6 on the small side and #2 on the large side in summer, depending on the hook style and I try to keep the fly in the big fish water. I consider a #8 very small for summer steelhead, and a 1/0 large. For muddlers I use a xlong #6 0r #4, summer bunny rats a #2 TMC, pupa flies #4 TMC 200R. I mostly use Tiemco hooks because of the high quality.
Just to summarize the presentation in the PNW is more like the classic atlantic salmon swing than a deep bottom drift and thus the larger flies are just the ticket for swimming the fly in a tantalizing manner across and down the fish's lie. For more background you could refer to some of the older British texts on tying low water patterns on really large hooks.