How to apply a function to a column of a Spark DataFrame?

You don't have to write a custom function because there is one:

import org.apache.spark.sql.functions.size

df.select(size($"tk"))

If you really want you can write an udf:

import org.apache.spark.sql.functions.udf

val size_ = udf((xs: Seq[String]) => xs.size)

or even create custom a expression but there is really no point in that.