Appears that the inner core is 750 mi in diameter, surrounded by an outer core that is 1400 mi thick.
Applying our volume formula:
Volume of core = 4/3 * pi * r^3
= 4/3 * pi * (1400 + 750/2)^3
= 4/3 * pi * 5592359375
= 23,425,252,581 As a percentage of the earth's volume, the core is
= 23,425,252,581 / 260,711,886,825
= 9%
Remember that the inner core is solid. If there is no motion present in the inner core, I don’t believe that it contributes to the creation of the magnetic field.