Hello Tijo

Crystal size can not be calculated from data. When you mount your crystal for data collection at that time time you may measure crystal size by using apex software and video camera.

I assume you mean powder XRD. Use the Scherrer equation, which relates the width at half maximum of XRD peaks to the crystallite size. The equation is d = K*lambda/[(fwhm)*cos theta], where K is a shape factor approximately equal to one, lambda is the Xray wavelength used, fwhm is the full width at half maximum of the peak (in radians, after subtracting instrument-related broadening), and theta is the angle of the diffraction peak. The value will be the size of the crystal domains that are diffracting, which may be smaller than the grain size. It only works for sub-micron size particles.

Follow Ellen Rice ans. and for more details search on web...lot of information are available on the web. Good luck