Algorithm which allows one to define image element sizes is suggested. It is based on statistical analysis of two-dimensional random fields and spectral density calculation. 
The result of image processing by this method is probability distribution function for the element sizes. The algorithm was tested and its effectiveness was shown in the work with model random fields. Scanning tunneling microscopy images of semiconductor nanostructures (quantum dots) were processed by this algorithm. Lateral sizes of quantum dots were estimated. The method can be also used in element shape description.