A grown Bernese Mountain Dog is usually 70 to 115 lbs and measures roughly 40 to 46 inches from nose to the base of the tail. Big-framed, thick-coated, and heavier through the front end than most people expect.
Berners are commonly affected by hip and elbow dysplasia and osteoarthritis. They also have a shorter life than most breeds and are considered senior early, often around six or seven. That's the part worth sitting with: the window where a supportive bed helps most arrives sooner for a Berner than it does for a Lab.
The coat matters too. A Berner carries a heavy double coat, sheds seasonally in serious quantity, and holds heat. A bed that traps her against a warm surface is a bed she'll abandon in summer.
Egg-crate orthopedic foam takes the load off the hips and elbows while she rests, and the cover comes off and washes, which, with this coat, you will be doing.