The size of the coop is important but there is a lot more to consider. I prefer a wooden coop with a roost and lots of ventilation. This certainly better than a wireframe cage for the nightly hours. During the day they need space to walk around and scratch. Take a sand bath …
Chickens that are kept in a poor situation often get stressed and sick. I do hope to you see you again here, to help and give more advice for happy chickens.
Yes, please give us some more information and pictures of you’re setup , the bedding, ventilation, you’re climate and the feed you give you’re Serama’s.