Both big and small, floating & sinking for Thompson
Thompson river steelhead are known to take small nymph-like patterns as small as size 6 or smaller, and big maribou popsicles, and a whole lot that falls in between.
I'd recommend
Doc Spratley 6, 4
(or other nymph like pattern such as black & grizzly wooly worm or Silver Hilton)
General Practitioner (in orange and black) 4, 2,
(or other prawn patterns, such as Squamish Poacher)
Maribou Popsicles (variety of bright and dark colors) 4, 2, 1/0
October Caddis 6,4,2 (or other high commotion waking flies)
As for lines, some of the runs fish best with a sink tip, or a cone head or weighted fly, but Thomson steelhead will come up for a dry fly even in wintry conditions.
A lot of the decisions of what pools and how you'll want to fish depend on the water level. There is a great chart on the effects of this in Art Lingren's Thomson River Journal put out by Frank Amato. He also has some interesting variations of Doc Spratleys and good history.
Vinnie in Juneau