How left out down-sampling in 3D-DWT just on the Z direction I am working on image processing by a  3D wavelet transform.

I have a problem with the classification size of the wavelet coefficients. As you know, when we apply 3D_DWT on the image (for example: Hyperion of the area by 256*256*128 size) the sub-band size would be decreased (for LLL in 1l level the size is 128*128*64). 

How I could apply down sampling just for the Z direction (spectral direction)?

It means this size of the sub-band should be 256*256*62 after one level wavelet applying. I want to do it in the Matlab script (dwt3.m &amp; wavedec3.m), but when I removed the down sampling, it was applied in all three directions.
You are applying a 3D DWT scheme whose down-sampling is coherent across the three  directions. Nothing forbids you from decomposing bands the way you want, and the literature is full of so-called isotropic, Tensor, Separable, Square or Standard implementations (vocabulary: separable/non-separable,
standard/non-standard, S-form/NS-form, rectangular/square, anisotropic/isotropic, tensor/Mallat, hyperbolic/isotropic, separated/combined wavelet transform)

To do that, it is more appropriate to perform 1D wavedec passes your own way on each direction separately.