For lines around 15lb, size 10 swivels will be more than up to the job. About size 7 will be more suitable if you want to use heavier snoods. For snap swivels, I use the american crosslock 100lb strength from Veals - I think they are about size 3.
I have always gone by the rule that the wire diameter of the swivel eye should be larger than the diameter of the line, to prevent the swivel weakening the line. Also, I make sure that the breaking strain of the swivel is at least twice the lowest line strength between the hook and the reel.