X-ray diffraction primarily measures regular arrangements of atoms. If the regular arrangement of atoms lead to a pore, then the pore size can be estimated from the crystal structure. In layered materials, and in particular if the 'top' and 'bottom' layers have the same structural arrangement, the pore size can then be deduced from the XRD-pattern.

how large are the pores?

Yes, My assumption is If the starting peak is higher than 8 degree that is small pore, in between 6 to 7 medium pore, 5 to 6 large pore 5 to 5.5 very large pore, lesser than 5 ie around 2.5 or still lesser will be mesopore. I hope these are reflected in most of the samples. However, this is not shown by all the samples. Why such deviation occurs. It is a matter of discussion.

N. Venkatathri is right, however, it very often disobeys the relationship. Specially when your first peak of XRD is small peak. It reflect the only dimension of unit cell of structure of zeolite. When the first peak is very strong, the most of case are follow the rules mentioned by N. Venkatathri. For example,ITQ-22 sample has its peak below 5, however, it is only 12*10*8 ring structure. ITQ-43 has very strong peak at around 4 degree. It is an extra-large pore zeolite with 28*12*12 channel system.

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If your sample having some porosity then you have to measure XRD at lower angel. After measuring the XRD, analyzed this data by using JCPDS data. From this softwere you known the unit cell structure of sample, also this data give (h,k,l) value and d spacing value. Use dx = 8M/ Na^3(means 'a' cube) , where 8 is number of molecule in unit cell of spinel lattice, M for M.W of sample , N is avogardo's number and 'a' is lattice constant, dx is X-ray density. Then for porosity (P) calculation use P= (1- ds/dx) where, ds is bulk density of sample this can be calculated from geometry of mass sample.

You must understand two different types of information obtainable from X-ray diffraction.

(1) The first robust information you can get comes only from the Bragg peak positions and this is about the lattice periodicity and the unit cell dimension. We do not need the intensity information at this stage. You can index the Bragg peaks from the cell dimensions. If your cell is centered then you will have some extinct reflections that is to say that some of the Bragg peaks hkl will have zero intensity. But we still do not need the intensities of each hkl reflections for these information. Careful indexing and checking for extinct reflections will thus give you the unit cell dimension and some symmetry of the lattice and if you are may be also the unique possible space group. So far we are still determining the lattice properties. But this does not give any information about the crystal structure.

(2) You must understand that lattice information although needed is not the final solution of the crystal structure which is a complete information of the atomic positions of the unit cell, in fact only the asymmetric part of the unit cell. The crystal structure can be thought of as lattice + basis. This simply means that we must know the positions of the atoms attached (mathematically speaking) to the lattice points. For determining this we must know intensities of all (as many as possible) hkl reflections. If we had the information of the phases of the scattered wave front then we could solve the atomic positions by inverse Fourier transformation but we do not have them usually. So you must play some tricks well described in standard book of X-ray crystallography. By these tricks you can solve the crystal structure. Then you know the positions of all atoms in the asymmetric unit cell and using space group symmetry transformation matrices you can get positions of all atoms in the crystal. You must check that the calculated atomic positions generate the Bragg reflections at the same positions and with almost equal intensities as you have determined experimentally by looking at the agreement factors. If this is satisfactory then you have successfully determined the crystal structure of your crystal.

You guys never bother to understand this basic ideas of crystallography but see just the help of some black box magic programs that do the job. So you do not understand these two important steps (the programs do that for you).

Now coming to your problem we can easily understand that the information of Bragg positions only give you lattice periodicity and can never give you pore sizes of the structure. This information needs the complete atomic coordinates, i.e. you need to know the complete information (1 + 2) described above. The intensity information leads to the crystal structure determinations, peak positions only give the mathematical description of the lattice only.

My advice to you: Before you try to solve structures and answer the questions that arise in materials science read text books on X-ray crystallography and do not go straightway to the data and the program. At least you can do these two things at the same time may be but you must read text books. Of course you people do not really solve any crystal structure but just refine a known structure and determine a slightly modified parameters because most of the structures have been determined by previous workers. But that does not mean that you do not need to understand at least the principles of crystal structure determination.