The effects of the excluded volume heterointeractions between dissimilar units on the conformational properties of miktoarm star copolymers A(x)B(f-x) in dilute solutions are studied by means of off-lattice Monte Carlo simulations. We calculate the dimensions of the two homopolymer parts and the whole molecule, the mean square distance between the two centers of masses, and other static properties for various x and f and solvent conditions. From these quantities, we calculate different expansion factors relative to homopolymer precursors. The results are analyzed considering recent renormalization group predictions for these copolymers. The differences with respect to previous results for Linear diblock copolymers, due to the influence of the star core effects, are also discussed.