Command which uses brackets automatically when applied to more symbols

Here's a version with a *-variant that uses \mleft and \mright or an optional argument for manually sizing the parentheses:

\documentclass{article}
\usepackage{xparse,mleftright}

\ExplSyntaxOn

% pass control to an inner function
\NewDocumentCommand{\astcommand}{sO{}m}
 {
  \IfBooleanTF{#1}
   { \cstoica_astcommand_auto:n { #3 } }
   { \cstoica_astcommand_manual:nn { #2 } { #3 } }
 }

% *-version: we want to use \mleft(...\mright)
\cs_new_protected:Npn \cstoica_astcommand_auto:n #1
 {
  \_cstoica_astcommand:nnn { \mleft( } { #1 } { \mright) }
 }

% normal version; #1 is either empty or \big, \Big, ...
\cs_new_protected:Npn \cstoica_astcommand_manual:nn #1 #2
 {
  \_cstoica_astcommand:nnn { \mathopen{#1(} } { #2 } { \mathclose{#1)} }
 }

% #1 is the left delimiter, #2 is the string, #3 is the right delimiter
\cs_new_protected:Npn \_cstoica_astcommand:nnn #1 #2 #3
 {
  \int_case:nnF { \tl_count:n { #2 } }
   {% only one token, no fences
    {1}{ #2^{*} }
   }
   { #1 #2 #3^{*} } % more than one token
 }
\ExplSyntaxOff

\begin{document}

$\astcommand{a}\quad
 \astcommand{ab}\quad
 \astcommand{\sqrt{a}}\quad
 \astcommand{}$

$\astcommand[\big]{a}\quad\astcommand[\big]{ab}$

$\displaystyle\astcommand*{\frac{a}{b}}$

\end{document}

Caveat An input such as \astcommand{{abc}} will result in no parentheses. To force parentheses, add an empty group like in

\astcommand{{}a}