What size crate is suitable for a cocker spaniel?

Depends really what you want it for and how long the dog will be in it.

I tend to go for one at least one size larger than that recommended by the manufacturers as I think they are too small. For comfort for the dog, if it is just one or even more, go for the largest one that you can accomodate. As a minimum, I would recommend 30"L x 24"W x 27" H. You can get away with one a bit smaller if it just to hold the dog for a short time rather than be a bed or retreat area.

hth