Making a shorter minus

I use \text{-} for a shorter minus sign.

Would something as simple as a \scalebox do?

\documentclass{article}
\usepackage{fixltx2e,graphicx,mathpazo}
\begin{document}
\( \sqrt{\scalebox{0.75}[1.0]{\( - \)}\frac{1}{9}} \)

% cf.
\( \sqrt{-\frac{1}{9}} \) 
\end{document}

Here, I am using the optional argument to \scalebox to set the vertical scaling to 1, so that only the horizontal size changes.

for a unary minus that isn't going to be surrounded by a lot of other math -- you did say this would be on a t-shirt -- you might try an en-dash. it's thinner and wider than a hyphen, but at about the same height off the baseline as a hyphen, so you might want to fiddle a bit with the vertical position.