You could try
text{\sqrt[\large N]{\dfrac{\lambda}{3^e}}
:
$\quad\displaystyle\sqrt[\large N]{\dfrac{\lambda}{3^e}}$
or text{\sqrt[\Large N]{\dfrac{\lambda}{3^e}}
:
$\quad\displaystyle\sqrt[\Large N]{\dfrac{\lambda}{3^e}}$
but \sqrt[\huge N]{\dfrac{\lambda}{3^e}}
:
$\quad\displaystyle\sqrt[\huge N]{\dfrac{\lambda}{3^e}}$
starts to look very odd.
Afterthought:
I was looking at the larger indices and they looked kind of crowded to the radical. I think these look even better:
text{\sqrt[\raise{2pt}{\large N}]{\dfrac{\lambda}{3^e}}
:
$\quad\displaystyle\sqrt[\raise{2pt}{\large N}]{\dfrac{\lambda}{3^e}}\quad\Large\star$
or text{\sqrt[\raise{2pt}{\Large N}]{\dfrac{\lambda}{3^e}}
:
$\quad\displaystyle\sqrt[\raise{2pt}{\Large N}]{\dfrac{\lambda}{3^e}}$
I think the starred one looks best