4" baits 1/0 or 2/0
5" & 6" baits 3/0 to 4/0
7" & 8" baits 5/0
That said just remember there's no hard and fast rules regarding hook size. Too small a hook and you could end up missing fish that bite the rear half of the bait. However, big and heavy gauge hooks can interfere with tha action of the bait. For small or suspending/ floating baits you wanna go with small light hooks to get the best action or float the bait off the ground.
super flukes use a 4/0 hook
super fluke jr use 2/0
Baby brush hog 2/0
6" dead ringer 2/0
4" powerworm 1/0 or 2/0
Play around with sizes and you'll figure out pretty soon what the best size is for a particular softbait.