Radiation box should be located atleast lambda/4 away from the outer surface and accordingly the dimensions can be determined.

Irrespective of antenna shape or type, the radiation box dimensions are as follows:

It should be between (lambda0)/2 to (lambda0/4). If you define more than that also no problem. But, computational time will increase. In fact, you will get more or less same results.

One more point is, the radiation box dimensions does not affect majorly on your return loss performance characteristics. The radiation box is only for radiation results.

If you have the latest HFSS, just right click and select create open region, one window will pop up, select radiation boundary, ok. The radiation boundary will be created automatically. Hope this helps

For better results lambda/4 is preferred