Re: far field by HFSS
sorry to disagree, aboozar.hamidipoor...
The size is radiation box is the bigger the better, as long as you have the necessary computing resource to handle it.
1meter from the antenna might not be far engough such that the field can be considered as "Far Field".
usually, the radiation box refers to the place where radiation boundary is applied. The radiation boundary might be artificial and/or even non-physical (such as close to the PML). So one needs to be mindful that the field computed on the surface of radiation box might not be the physical field (as radiated by the antenna).