problem in metamaterial simulation

i simulate a SRR unit cell in HFSS but when I change the height of the radiation box ,result have change so what should be the height of the box .Please help me ,it is very urgent.

I guess, you made a mistake in Setting up the boundary conditions.
I used half a wavelength above and below the SRR/Substrate.
How can that be urgent, it has been published extensively in the past 6-8 years.
Search the edaboard for fss and check the entries 2005 to 2007. I'm pretty sure, we discussed this topic by that time.

There are numerous *spurious* modes which exist on the fast-wave side of the light-line, whose frequencies scale with the height of the box/boundaries.

If you are sure you have the correct mode, then cd79 is correct that you've most likely made a setup error.

In addition to everything that's been mentioned, you should be using Floquet ports to excite the unit cell. After that, it's simply a matter of solving for the appropriate number of modes. This will be displayed in the modes calculator, and will vary based upon frequency, unit cell size, and scan angle.