Dear José,

Spot size is determined by the diffractometer setup, so only you can know it for your own device. In the most usual case of the Bragg-Brentano geomery with fixied divergence slit the spot will have rectangular shape. Its width will be constant (usually between 10 and 20 mm). And its length will be determined by your choice of the slit width and goniometer radius. Also, spot length will be different at the different incident angles (but it is usually assumed that penetration depth will also change, so that the irradiated volume remains the same).

You can refer to the nice presentation prepared by Scott A. Speakman in MIT for more details.

http://prism.mit.edu/xray/oldsite/Basics%20of%20X-Ray%20Powder%20Diffraction.pdf

Right. The spot size is determined by your own set-up parameters (slit dimensions, the divergence in your mirror system including goniometer radius, etc.).

You can observe the spot on a fluorescent target placed as a sample when you darken your experiment room (event it is difficult). 

place fluorescent tap instead of your your sample and run the X-rays . It will burn the portion of  tap on which X-rays had fallen. Measure the burnt area which will be your spot size.

Can we simulate an x-ray spot size on an area detector? Is there a such package to do?