The study of planetary magnetic fields can provide important information on a planet's interior structure and composition. Here we use numerical dynamo models to demonstrate that it is possible to determine information on a fundamental core property: inner core size, from observations of intense, small-scale radial magnetic flux patches. The inner core size is discernable from the boundary between different types of intense magnetic flux patches inside and outside the inner core tangent cylinder. This type of study could help in determining the solid inner core size in Mercury, Ganymede, Jupiter and Saturn. For Mercury and Ganymede, the inner core size would then provide useful constraints on thermal evolution models for these bodies.