As far as I understand, fractal is an object/geometrical shape, and fractal dimension is an factor which describe this object (describe how the space is filled by the object) - form pure "mathematical" fractals it can be calculated, for other shapes, like e.g. Cells etc, fractal dimension can be estimated using various methods, like e.g. Box counting method.
based on my research you will have a log-log plot, number of pores on y axis and pore size (radius) on y axis and you will get the fractal dimension from the slope of the straight line.
There are several definitions of the "fractal dimension", and several "fractal measures". I would strongly advise you to go by the book "Chaos in dynamical systems"