Unlike larger breeds, springers tend to curl up when they sleep and and don't take up a lot of space, so a large, covered, foam-rubber filled, pillow-type dog bed with a faux sheepskin cover will work. These are readily available at most pet stores and will work well.
The choices for dog beds are truly endless, but can be narrowed down by looking at online catalogs or by shopping in a pet store to see the beds up close. Size can be the biggest consideration, as too big is almost as bad as too small. Springer's like to be cozy.
A springer spaniel needs something durable and comfortable. They are energetic and often like to play with just about anything. I recommend finding a bed that will be in a stationary location so that the dog will be unable to move it, and will have a single place to lie when it's bed time.
Most pet stores have a chart but you should also look at your dog's sleeping style. Does your dog sleep curled in a ball? Spread out? Is he arthritic? Does he like to make a, "Nest" out of pillows? Take this information with you to the pet store. They can help you choose the right bed.
Try to find a mid sized dog bed for your springer spaniel. Get a bed that allows your dog to wiggle around and get comfortable. Also look for a bed with lots of soft cushion and padding. Because of this breeds energy level they need a nice soft and relaxing place to crash.