periodic boundaries

Will

I am simuilating a combustor section in an annulus combustor. I read in Fluent manual that to check periodic boundaries use grid/check and that teh difference between the minimum, maximum, and average should have negligible values and if they do not then something is wrong with grid. Now if you view entire geometry with periodic repeats then the entire geometry is very large, but I am only simulating a section of combustor. Now my model does not show a change in the max,min, and average in grid check. What can be wrong? Could it be that my entire geomtry is so large? or that my section is small? thanks