The best backpack for the Wonderland Trail is a self-supported multi-day pack in the 55 to 70 liter range, because this roughly 93-mile loop around Mount Rainier is not portered and has limited resupply. Unlike the world's teahouse and hut treks, you carry your own shelter, sleep system, and several days of food, so the Wonderland is closer to a domestic thru-hike than a portered trek.
Why it needs more volume
Most hikers walk the Wonderland in 7 to 10 days and use one or two cached or ranger-station resupplies, so the longest food carry can be four or more days. Add a bear-resistant food storage requirement, frequent rain, and river-crossing gear, and a 60-to-65-liter pack with a supportive frame becomes the sensible choice. Ultralight hikers with low base weights manage in 50 to 55 liters.
Wet-weather setup
Rainier makes its own weather, so waterproofing matters more here than on a dry trek. Combine a rain cover with a pack liner, and choose a pack with a stable hipbelt for the constant elevation change. Because it is self-supported, size the pack the way you would for any multi-day backpacking trip: to your torso first, then to your longest food carry.