All sizes can be used as mainline to leader connections for any kind of drift/float fishing applications, with increasing weight to allow for greater depth in heavy current/tidal flows. The #8 power swivel is .2 grams equivalent to #4 shot for float fishing.
The recommended swivel size for 45lb or 32lb copper, Leadcore, and Weighted Steel to backer connections is #8 or #7.
The recommended swivel size for Walleye copper to backer/leader is #10.
The recommended swivel size for Walleye braid jigging to fluorocarbon leader connections is #12.