Prepare the following input file:
\documentclass[12pt]{article}
\usepackage{helvet}
\begin{document}
\newlength{\cmheight}
\newlength{\helvheight}
\settoheight\cmheight{x}
\settoheight\helvheight{\sffamily x}
\fpeval{round(\cmheight/\helvheight,5)}
\settoheight\cmheight{A}
\settoheight\helvheight{\sffamily A}
\fpeval{round(\cmheight/\helvheight,5)}
\end{document}
You may need \usepackage{xfp}
if you aren't running the most recent LaTeX release.
Upon running LaTeX you'll get
0.8272
0.9458
These are the scale factors for the two cases you may want: in the first case the x-height of Helvetica will be the same as Computer Modern; in the second case the height of capital letters will be the same.
Indeed, in Helvetica the ratio between x-height and capital letter height is different than in Computer Modern.
Now let's make a couple of experiments.
First experiment: same x-height
\documentclass[12pt]{article}
\usepackage[scaled=0.8272]{helvet}
\begin{document}
Abcdef\textsf{Abcdef}
x\textsf{x}
\end{document}
Second experiment: same capital letter height
\documentclass[12pt]{article}
\usepackage[scaled=0.9458]{helvet}
\begin{document}
Abcdef\textsf{Abcdef}
x\textsf{x}
\end{document}
Conclusion
If you want all the document in Helvetica, decide whatever scaling you seem fit, between the two given bounds.
I'd probably go with 0.94
\documentclass[12pt,oneside]{article}
\usepackage[scaled=0.9]{helvet}
or whatever