The following code comes from this post. The problem is that I would like a way to automate the size of the box so as to be very small with a few of spacing, but the following solution doesn't work with fractions for example.
\documentclass[border=5pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{calc,shapes}
\newcommand{\tikzmark}[1]{\tikz[overlay,remember picture] \node (#1) {};}
\newcommand{\DrawBox}[1][red]{%
\tikz[overlay,remember picture]{
\draw[#1]
($(bl)+(-0.2em,0.9em)$) rectangle
($(br)+(0.2em,-0.3em)$);}
}
\newcommand{\MyBox}[2][red]{\tikzmark{bl}#2\tikzmark{br}\DrawBox[#1]}
\begin{document}
Lorem ipsum dolor sit amet. \MyBox{Lorem ipsum} dolor sit amet. Lorem ipsum dolor sit amet.
Lorem ipsum dolor sit amet. \MyBox{$\frac{\frac{2}{5} - 7}{4 + x^3}$} Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet. Lorem
ipsum \MyBox[blue]{$3.29 \times 10^{29}$} dolor sit amet. Lorem ipsum dolor sit amet. Lorem
ipsum dolor sit amet. Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet. Lorem
\MyBox[draw=red,fill=yellow!20,,opacity=0.3]{$3.29 \times 10^{29}$} ipsum dolor sit amet.
Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet.
\end{document}


