Domestic Long Hair isn't a breed. It's the label used for a mixed-ancestry cat with a long coat, the fluffy counterpart of the Domestic Short Hair. These cats come from the general cat population, so you'll see every color and pattern, from tabby and calico to solid black, smoke and bicolor. Some look a lot like a Maine Coon or Persian without sharing that ancestry.
With no breed standard, size, coat texture and personality vary from cat to cat. Most adults weigh 8 to 15 pounds, and long fur can make them look even bigger. What they share is a coat that needs regular brushing to prevent mats and hairballs. Mixed ancestry also tends to bring broad genetic diversity, which is one reason many of these cats stay healthy well into their teens.