Libertinus Math: (too) small math symbols?

You load those two symbols with a different Scale.

\documentclass{article}
\usepackage{unicode-math}
\setmathfont{Libertinus Math}
\begin{document}

$A\otimes B$ and $A\oplus B$.

\setmathfont[range={`⊕,`⊗},Scale=1.2]{Libertinus Math}
%         or range={\oplus,\otimes}
%         or range={"2295,"2297}
$A\otimes B$ and $A\oplus B$.

\end{document}

I agree that those symbols are ugly.

You can borrow glyphs from other math fonts. Here's XITS Math (but scaled down a bit):

\documentclass{article}

\usepackage{unicode-math}
\setmainfont{Libertinus Serif}
\setmathfont{Libertinus Math}
\setmathfont{XITS Math}[
  range={\oplus,\otimes},
  Scale=0.8,
]
\setmathfont{Libertinus Math}[range=] % restore the proper font dimens

\begin{document}

$A\otimes B$ and $A\oplus B$.

\end{document}

It is customary to have the symbols a bit below the baseline (compare with the standard Computer Modern fonts) so their geometric center is on the formula axis.

You can try also creating the symbol with tikz and to add your preamble.

\documentclass{article}
\usepackage{tikz}
\newcommand{\Bigotimes}[1]{
\begin{tikzpicture}[#1]
\draw [line width=0.1pt] (0.,0.) circle (1.cm);
\draw [line width=0.1pt] (0.,1.)-- (0.,-1.);
\draw [line width=0.1pt] (-1.,0.)-- (1.,0.);
\end{tikzpicture}
}
\begin{document}
We have $A\Bigotimes{scale=0.2}B$ where you can decrease or increase the symbol with the option scale=0.2.
\end{document}