Daybeds are usually referred to a smaller beds that are placed against the wall and usually are meant for kids' day sleep. They all use twin mattresses as default. Mattresses, btw, are not a part of daybed, you would need to buy that separately. Some additional pieces as daybed platform might be required for some beds (we usually have a note about that at the product description). Some daybeds have additional storage bed under the main one - in this case you may purchase a secondary mattress for that as well.
Daybeds are mostly meant for kids, teens and young adults. However, there are no size limitations for most of them. Daybeds are usually not as heavy/solid as regular beds, so some overweight person might better use a normal bed instead.