Minimum length for \xrightarrow

In such cases, the command \makebox comes handy. It puts the argument text into a box of customizable width. Even more useful in math mode is \mathmakebox provided by the mathtools package, because it's working better in math mode and respects the current math style.

So, we could start with \xrightarrow{\mathmakebox[3em]{1}}. But we can do better - let's write a macro for that, which produces an arrow of the same width like a text we specify. Here's such a complete example:

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}
\newlength{\arrow}
\settowidth{\arrow}{\scriptsize$1000$}
\newcommand*{\myrightarrow}[1]{\xrightarrow{\mathmakebox[\arrow]{#1}}}
\begin{document}
$\myrightarrow{1}$

$\myrightarrow{1000}$
\end{document}