The X-ray diffraction intensity is in proportional proportional to the crystal volume and therefore also crystal mass because larger the volume or mass the larger the number of scattering atoms or scattering centers. Well here we assume that the absorption effect is negligible which is of course not true always. Also we assume that the X-ray beam size is larger than the crystal size. With normal X-ray single crystal diffractometry with Cu, Mo or Ag targets one should choose a small single crystal of the order of about 0.1 mm in linear dimension. This size will enable you to get good set of data for structure refinements because the absorption and extinction with be small. Also it is nice to use a single crystal of well defined size with known crystal faces or a spherical crystal if you can make it.  With high energy X-ray diffraction with synchrotron radiation one can use bigger crystals of size of a few mm. For neutron diffraction on the other hand you need a crystal of the size of a few mm say 2x2x3 mm^3. With medium energy X-ray diffraction at a synchrotron X-ray source you can use crystals of micron size and still get good data set.

For quantitative expression of X-ray diffraction intensity read B.E. Warren, X-ray Diffraction, Dover (1990).

For the same crystal higher intensity means larger crystal size but one must consider the absorption effects.

 I would like to give you an example of the observations made by Dr. Tapan Chatterji: calcite crystals (CaCO3) has a very ordered structure, resulting in well defined peaks even it is small in size than its polymorph,vaterite (CaCO3, too). I normally prefer to measure crystal size distribution by other technique, than to correlate with x-Ray diffraction. These techniques may include laser diffraction or even electron microscopy for the estimation the size of few crystals.  

If V is the volume of the crystal embedded in the X-ray beam,  the transmission factor T can be calculated from the following expression

T=1/V integralV exp(-mu(p+q)) dv

where the integration is over the volume V, mu is the linear absorption coefficient, p and q are the paths of the incident, resp. diffracted beam to the volume element dv.

mu can be calculated from the individual mass absorption coefficient of each atom included in the chemical formula.

The intensity of a diffracted beam is thus reduced by a factor T relative to a (hypothetical) non absorbing crystal.

Your question is interesting but  too much general. What do you mean with crystal size?  The size of a single crystal  or the mean size of the crystallites (as it occurs for x-ray powder diagram) ?.  Are you interested to compare the diffracted intensity by different crystalline substances or to compare the diffracted intensity by the same crystalline substance with different size distribution of the individuals embedded in the x-ray beam ? In the latter case the answer by G. Chapuis and T. Chatterji are clear enough to solve your problem. In the former case it is worth remembering that the diffracted intensity  is much more influenced by the atomic diffusion coefficient (ruling the structure factor) than by the crystal size

what is an atomic diffusion coefficient and what does it have to do with structure factors?

To answer your question... it depends if you are doing powder or single crystal measurements. In the case of single crystals then yes the scattering intensity is directly related to the crystal volume being illuminated. For a powder sample you would have to do small angle scattering to determine the size distribution of the grains.