\widetilde is too big and \tilde is too small -- is there something between?

It's a bit hacky, but does

\usepackage{amsmath}
$\overset{\sim}{\mathcal{W}}$

(which gives ) do what you're after?

Edit:

An alternative with adjustable height is

\newcommand\Wtilde{\stackrel{\sim}{\smash{\mathcal{W}}\rule{0pt}{1.1ex}}}

where you can change 1.1ex to whatever you like.

Note, however, that in this implementation the tilde height is entirely independent of the size of the original W, so you may need to define several versions if you want to do this to lots of different letters.

Follow Ant's solution:

\documentclass{article}
\usepackage{accents}
\begin{document}

$\tilde W \widetilde W$

$\accentset{\sim}{W} \accentset{\textstyle\sim}{W}$

\end{document}

---

That's not very good. For more complicated solution, see How can I manually choose the size of a wide accent? (math mode)