What were the most popular lengths for baby names in 2015?
For girl names, the top length was 6 letters, followed by 5 letters and 7 letters:

The top lengths were the same for boy names:

And here are both genders side-by-side so you can compare:

We saw the same top three lengths for baby names in 2014.
U.S. Baby Names 2015: Most Popular Baby Names, Top Debuts: Girl Names, Top Debuts: Boy Names, Biggest Changes in Popularity: Girl Names, Biggest Changes in Popularity: Boy Names, First Letter Popularity, Name Length Popularity