Australia is clearly the larger of the two: about 7,706,009 km² km² against 16,879,701 km² km², roughly 2.2× at true size. Drag one onto the other on the map above to feel the gap.
Australia sits in Oceania, Russia in Europe. On the old Mercator map the latitude gap distorted how large each felt beside the other; at true size the answer is simply 2.2×.
The old map muddied it further: Mercator inflates Australia about 1.2× and Russia about 6.1×, so on a Mercator-style map this pair appears 10.8× apart instead of the true 2.2×.
Australia fits 2× into Russia.