That information is not recorded after the end of the procedure.
If you passed an options structure in to ga, then use
gaoptimset(TheOptionsStructure)
to see what the final options would have been.
If you did not pass an options structure to ga, then use
gaoptimset(@ga)
to see what the options used were.
PopulationSize is not dynamic: it is determined by the options passed in or defaulted to.