Answer
Longer.
Work Step by Step
The frequency of a standing wave is defined as $f=\frac{nv}{2L}$. Using $v=f \lambda$ to relate frequency and wavelength, $v=\frac{nv \lambda}{2L}$, or $\lambda = \frac{2L}{n}$, after cancelling out terms.
So, if the length of the string remains constant and the harmonic number decreases, the resonant wavelength must be longer.
