My Cochins, standard and bantam, hatch anything. I have put tiny little bantam eggs under the standards and I have put big Marans eggs under the bantams. they hatch and raise the chicks very well. Also it depends on nests and environment. I give my girls roomy nests in a secluded corner without any other hens to get to them.
A hen is broody when she stays on the nest for 24 hours straight and has the trance look to her eyes. Kinda blank looking even when you are moving around inside the coop. If you try to touch her she will often fluff up and give you the dinosaur scream. Different than when you are just disturbing the egg laying process.
I never take a new hen serious about being broody until I can give her some real eggs to hatch and be able to move her at night to her broody location, away from other birds and usually a small pen or dog crate by herself and she stays there.
Personally, I have liked my Cochins brooding the eggs better than my Silkies. My Silkies often let the eggs get out from under them and they get chilled. Doesn't seem to matter what size of egg, there are always one or two that get chilled and do not hatch. My Cochins flop their big fuzzy butts on those eggs and sit for a good 3 weeks while making sure every egg is turned and talked to. And my Cochins have made better mamas than the Silkies. The Silkies seem to lose the chicks and not care if they are following or not.