According to the Missouri Botanical Garden, P. 'Laura' grows 3-4 ft. tall. See link below for more detailed plant/growing information. My tall phlox all took 3-4 years to get established. Most were planted in 2011; they were splendid this year.

Here is a link that might be useful: Phlox paniculata 'Laura'

Perhaps there's some confusion between 'Laura' and 'Little Laura', latter is said to be 18 to 24 inches tall.

Have used both for a good few years. They are very hardy and long-lived here.

The other thing is that factors related to location in the garden may affect garden phlox height attained in any one year (soil fertility, sun, etc.).

Taking time to reach their potential height and (later) requiring division are both potential factors.