After many years of drop shotting, I've settled on a size #2 Gamakatsu Split Shot/Drop Shot hook. This goes for LM or SM bass. Through trial and error, I found that size #4 is just a tad too small (but they will work sometimes) and size #1 are too large, overpowering your bait. A size #2 is a good compromise.
A general rule of thumb - if there ever could be one - would say something like, "use the largest hook you can get away with, without over powering your offering".
Now here's a most important design issue you need to consider. Compare the Gammy hook above with any other drop shot hook on the market. Look closely at the points and you will see that the Gammy's point is straight, with no curve or beaking to it.
I believe this gives you better penetration, when using a slow sweep, fast reel, type of hook set needed with the drop shotting technique. JMO.