How does one choose kosher hadasim when purchasing the arbat haminim? The Shulhan Aruch (646) discusses the requirements of the hadas.
The most important halacha relates to the clusters of leaves along the stem of the hadas. Each cluster much have three, symmetrical leaves, on each horizontal line. This law is called "meshulshim." R. Ovadia Yosef (Hazon Ovadia, Sukkot pg. 305) mentions an additional level of hidur. He writers that the leaves on the bottom cluster should preferably overlap with the leaves of the cluster above it, as it is a "misvah min hamuvhar" that the leaves should completely cover the stem. Of course, as long as the hadasim are meshulash, for the majority of the "shiur" (minimum length of the hadasim), the hadasim are valid.
What is the shiur of the hadas? The hadas should be at least three tefahim (handbreadths). A tefah is a measurement used to measure an amah- an arms length. There is a debate whether an amah is five or six tefahim. Maran writes that we assume an amah is five tefahim, and therefore three tefahim equal 10 agudalim, which is approximately 20 centimeters, or 7.8 inches. That would be the minimum length of the hadas which should be meshulash.
However, some poskim insist that an amah is six tefahim, and therefore the measurement would be 24 centimeters or 9.36 inches. While these measurements are according to R. Haim Naeh, the Hazon Ish held larger calculations, according to which the hadas should be at least 30 centimeters or 11.5 inches.
This is the misvah min hamuvhar. What if some of the leaves shed during the hag? What is the minimum amount of meshulash? As long as the majority of the minimum halachic length (shiur) of the hadas is meshulash, the hadas is valid and one may recite the blessing over the arbat haminim.
Summary: The misvah min hamuvhar is to use hadasim which are meshulash for the entire shiur of three tefahim (between 7.8 11.5 inches). However, as long as the majority of that amount is meshulash, the hadasim are valid. In other words, if a minority of the rows are not properly symmetrical (one leaf below or above the other two), or if one out of every three rows is missing a leaf, the hadasim are kosher.