define ferrite

Hi,all
I am an HFSS beginning learner,A question puzzled me when i reading about Ansoft HFSS-Material Manager about Ferrite Materials and Ports,as follows:
When designing a problem containing ferrite materials and ports, do not arrange the port so that it touches the ferrite material. If you must place a port on a ferrite material, separate the two with a dielectric with a relative permittivity equal to the relative permittivity of the ferrite.

The question is: how to know the size of the dielectric?

I am looking forward to your reply.

hi
the size of the dielectric must be very small.