periodic structure in feko

Hi all
I want to simulate a doubly periodic structure in FEKO. For the first simulation I want to simulate a PEC rectangular plate and set double periodic BC for that.
But I confused when I saw RCS of this structure changed when I changed the size of plate.I think periodic plate must act as a infinite plate and so the size of one cell should not affect the RCS quantity.
Can anyone help me?

I apologized for my delay.
attached file, is useful to simulate 1 2-D or 1-D periodic structure in Feko. or
Periodic Analysis — FEKO