There is no definitive answer to this question as it depends on a number of factors including the type of fish you are targeting the size of the waves and the water depth. In general however most anglers use a bucktail that is between 3 and 6 inches long.
If you are fishing in shallow water then a shorter bucktail will be sufficient. The shorter length will allow you to keep your bait close to the bottom where the fish are likely to be feeding. In deeper water a longer bucktail will be necessary in order to keep your bait down in the strike zone.
The type of fish you are targeting will also play a role in choosing the right size bucktail. If you are targeting smaller fish then a smaller bait will be necessary. However if you are targeting larger fish then a larger bait will be necessary in order to entice them to bite.
In general the size of the waves will also dictate the size of the bucktail you use. In larger waves a larger bucktail will be necessary in order to keep your bait from being swept away. In smaller waves a smaller bucktail will suffice.
Finally the depth of the water will also affect the size of the bucktail you use. In shallower water a shorter bucktail will be necessary so that you don’t waste time casting your bait out too far. In deeper water a longer bucktail will be necessary in order to reach the fish that are swimming at the bottom of the water column.
No matter what size bucktail you use be sure to match it with the correct size hook. A too-small hook will result in lost fish while a too-large hook will make it difficult to bait your hook properly.
When in doubt it is always best to ask the advice of a local bait and tackle shop. They will be able to help you choose the right size bucktail for the conditions you will be fishing in.