Smaller than the max size for doodling, and rough painting with hatches.
The max brush radius is not indicative of the actual brush size, feel free to customize the second brush size as you feel like.
I like to use it as a doodling brush to achieve thinner lines without being distracted and constrained by the cursor, its also very good for hatching and adding color variations in painting as it doesn't reach max opacity without maximum pen pressure ( I recommend not changing this, if you want max opacity its better to use another brush).