For a dog that weighs 55 pounds, a medium to large-sized dog bed would likely be the best fit. Here are a few guidelines to help you choose:
Medium Dog Bed: Typically suitable for dogs weighing between 30 to 60 pounds. A medium bed usually measures around 30" x 40".
Large Dog Bed: Designed for dogs weighing between 60 to 90 pounds. A large bed usually measures around 36" x 48".
Given that your dog is 55 pounds, a medium-sized bed should be sufficient, but if your dog likes to stretch out or if you want to provide extra space, you might consider a large bed. It's also a good idea to measure your dog's length from nose to base of the tail and compare it to the bed dimensions to ensure a comfortable fit.