I keep two bantam Plymouth Rock hens in the coop with my standard sized hens. They do just fine. What breed do you have?
Most bantam can fly/flutter as well if not better than their standard counterparts. Mine have no problem getting up on my roosts and I've even found my littles in the rafters of my run. I have no ramps. They use the same nest boxes as the big girls. Sometimes I'll find 2 littles side by side in the same nest. You will need to make sure that waterers and feeders are at the appropriate height. I have a block next to my nipple waterer so the littles can step up on it to reach. If you are housing just bantams you should be fine.
If you are housing bantams in with standard hens then you need to maintain extra square footage per bird, some escape routes/hiding places, addition roost/perch options, and additional feed and water stations in the coop and run to prevent the standards from being able to bully the bantams.