Other than meeting certain shape and minimum size requirements, lulavim are not generally graded by length or girth.
The lulav is the unopened frond or “palm spear” that grows from the top of the palm tree. All palm trees (and also grasses) have at least one lulav. Each lulav leaf has two parts which will eventually open up and fan out to form the palm branch that can be used to provide a cooling breeze. As the tree grows, the lulavim open out and splay outwards, making room for fresh lulavim that are at the top of the tree as it grows upwards.
What is important is the middle leaf – called the “teyomes”. Ideally this should not be split, but as long as the middle leaf is not split more than about 3″ it is perfectly kosher, even more than that they can be used in some cases. A middle leaf that is split less than 3″ can be glued together (e.g. with Elmer’s glue) to stop it splitting further, and it is perfectly kosher.
We mostly grade lulavim based on how closed or open the middle leaf is. Some lulavim are more slender than others. When they grow they can be six feet or longer. For practical purposes we need to be able to ship them both in bulk and also to individual customers.
We receive lulavim which are between about 36-42″ long. Sometimes we need to trim them from the bottom to be able to fit them into our shipping boxes. This will result in the outer most leaves falling away, but this is of no halachic relevance, since we are concerned with the middle leaf. The result of this is that you may receive a slender lulav, but hopefully one whose middle leaf is more closed.
Other than the above physical requirements a lulav must come from a palm tree that produces edible dates. There are many varieties of dates and lulavim from the “Deglet Noor” variety often have middle leaves that split easily. The Medjool date palm produces excellent lulavim, and those are the ones we supply most of the time. The “Deri” (Dayri) date palm produces spectacular lulavim whose middle leaves are closed and stay closed. We do supply these also, but they are expensive.
For a more comprehensive discussion of the kashrus of a lulav (as well as the other species) I recommend three books:
Summary of Halachos of the Four Minim by Rabbi Shimon Eider (Feldheim)