What is the correct size split ring to use on crank baits? Most of the ones I have a shallow runners.

There is no answer that will be right for all baits. Most of the crankbaits I use come with size 2 split rings. When I have to replace them I go to a size 3. Size 2 is too small and hard to work with for me even with the split ring pliers I use.

Many smaller bass cranks such as the squarebill KVD 1.5's are paired with #2 rings. I would go with #3's on anything larger up to maybe a 4" bait. Hope this helps. 

I only use #3s. Owner Hyper Wire .

I would say #1 or #2 for majority but may vary depending on lure/hook size. If you buying bulk size, look on ebay under $4 for a 100.