I have mostly Serama crossbreeds, and I house them in the same kind of coop you'd use for standard size hens, giving them the same minimum square feet indoors and out (4 square feet per bird inside, plus 10 square feet per bird outside). Seramas are small, sure, but they're lively and active birds and use the space you give them.
Seramas aren't known to be cold hardy, so you do have to consider supplemental heating in winter depending on your climate. And like other bantams, you can use a narrower roost than for big chickens. I use roosts that are about 1 3/4 " wide.