Q: If I have both bantam and large size hens, what size rooster should I get?
A: The answer to that question will depend on exactly how large the hens and roosters in question may be as well as what your personal preference is. No matter the size of your rooster, he will try to mate all your hens, whether they are large fowl or bantams, so it's certainly a valid concern! Look at the difference in size between a large fowl cochin and a bantam cochin:
If you have a very large rooster like a Jersey Giant paired with very small bantams like Sebrights, you might see some injuries during mating since he will be so large and heavy for her to bear. However, the reverse is also true: a very small bantam rooster will have to be fairly brutal to be successful in mating a large hen, and will often injure her comb as he holds on for dear life.
To minimize the problem, you may want to get a smallish, non-bantam rooster for your mixed-size flock, like a Hamburg, Leghorn, Polish, Dominique, Easter Egger, Campine or Andalusian.