How can you make a macron less wide?

You can define a short one that matches the longer ones and fits quite well on the i like this:

\newcommand{\shortmacron}{%
   \makebox[0pt][l]{%
      \rule[1.3ex]{0.2em}{0.035em}%
   }%
}

Another way is to define a smart version that automatically adapts it’s length. I also set this one a little lower to minimize the conflict with the drop serif of the f

\newlength\tmp
\newcommand{\smartmacron}[1]{%
   \settowidth{\tmp}{#1}%
   \makebox[\tmp][c]{%
      \rule[1.2ex]{0.6\tmp}{0.035em}%
   }\kern-\tmp#1%
}

Using the xstring package to check which letter follows, we can define the macro even more smart and align the macron better on top of the letter:

\usepackage{xstring}
\newlength\tmpa
\newlength\tmpb
\newlength\tmpc
\newcommand{\supersmartmacron}[1]{%
   \settowidth{\tmpa}{#1}% width of letter
   \setlength{\tmpb}{0em}% x-shift
   \setlength{\tmpc}{1.23ex}% raise
   \IfStrEqCase{#1}{%
      {a} {\setlength{\tmpb}{-0.04em}}%
      {e} {\setlength{\tmpb}{-0.01em}}%
      {i} {\setlength{\tmpb}{-0.045em}\setlength{\tmpc}{1.8ex}}%
      {\i}{\setlength{\tmpb}{-0.045em}}%
      {o} {\setlength{\tmpb}{-0.01em}}%
      {u} {\setlength{\tmpb}{-0.04em}}%
   }
   \makebox[\tmpa][c]{%
      \kern\tmpb\rule[\tmpc]{0.6\tmpa}{0.035em}%
   }\kern-\tmpa#1%
}

Of curse the right values for the lengthen \tmpa, \tmpb and \tmpc depend on the selected typeface.

Result and demo code
(original macron, short macron, smart macron, super smart macron)

\documentclass{article}

\newcommand{\shortmacron}{%
   \makebox[0pt][l]{%
      \rule[1.3ex]{0.2em}{0.035em}%
   }%
}

\newlength\tmp
\newcommand{\smartmacron}[1]{%
   \settowidth{\tmp}{#1}%
   \makebox[\tmp][c]{%
      \rule[1.2ex]{0.6\tmp}{0.035em}%
   }\kern-\tmp#1%
}

\usepackage{xstring}
\newlength\tmpa
\newlength\tmpb
\newlength\tmpc
\newcommand{\supersmartmacron}[1]{%
   \settowidth{\tmpa}{#1}% width of letter
   \setlength{\tmpb}{0em}% x-shift
   \setlength{\tmpc}{1.23ex}% raise
   \IfStrEqCase{#1}{%
      {a} {\setlength{\tmpb}{-0.04em}}%
      {e} {\setlength{\tmpb}{-0.01em}}%
      {i} {\setlength{\tmpb}{-0.045em}\setlength{\tmpc}{1.8ex}}%
      {\i}{\setlength{\tmpb}{-0.045em}}%
      {o} {\setlength{\tmpb}{-0.01em}}%
      {u} {\setlength{\tmpb}{-0.04em}}%
   }
   \makebox[\tmpa][c]{%
      \kern\tmpb\rule[\tmpc]{0.6\tmpa}{0.035em}%
   }\kern-\tmpa#1%
}

\begin{document}
%\huge
%\tiny
Tac\=e, M\=arc\=o, \=unus, f\=\i li\=\i?

Tac\shortmacron e, M\shortmacron arc\shortmacron o, \shortmacron unus, f\shortmacron\i li\shortmacron\i?

Tac\smartmacron e, M\smartmacron arc\smartmacron o, \smartmacron unus, f\smartmacron\i li\smartmacron\i?

Tac\supersmartmacron e, M\supersmartmacron arc\supersmartmacron o, \supersmartmacron unus, f\supersmartmacron\i li\supersmartmacron\i?

\=a \shortmacron a

\=a \smartmacron a

\=a \supersmartmacron a\quad
\=e \supersmartmacron e\quad
\=i \supersmartmacron i\quad
\=\i \supersmartmacron \i\quad
\=o \supersmartmacron o\quad
\=u \supersmartmacron u\quad

\supersmartmacron a\quad
\supersmartmacron e\quad
\supersmartmacron i\quad
\supersmartmacron \i\quad
\supersmartmacron o\quad
\supersmartmacron u\quad
\end{document}

---

To use the shortcut with on of the smart versions call

\let\=\supersmartmacron

in your preamble.