Pi appears very small with newtxmath scaling

Question

Generally i dont like the symbol pi given in the newpxmath package. So i tried to use the cmroman pi using the following command. But then due pi appears too small since i scale the text and math. Any idea how to scale the pi symbol??

   \documentclass[12pt]{article}
   \usepackage[margin=1in]{geometry}
    \usepackage[T1]{fontenc}
     \usepackage[scaled=1.2]{newtxtext}
      \usepackage[scaled=1.2,cmbraces,varbb]{newpxmath}

   \DeclareSymbolFont{lettersCM}{OML}{cmm} {m}{it}
    \SetSymbolFont{lettersCM}{bold}{OML}{cmm} {b}{it}
      \let\txpi\pi
    \DeclareMathSymbol{\pi}{\mathord}{lettersCM}{"19}
    \begin{document}
    \begin{flushleft}
    Pie appears very small
   $$\frac{k\pi}{12} = \frac{22}{7}$$
     \end{flushleft}
    \end{document}

and the output appears like this

Answer 1

Don't scale the fonts. Your main font size is 14pt (precisely 14.4pt) and it's much better, if you really want such humongous size, to use a proper package/class like extarticle.

For the comparison below I used your exact code, just adding a lipsum paragraph. The extarticle version is

\documentclass[14pt]{extarticle}
\usepackage[margin=1in]{geometry}
\usepackage[T1]{fontenc}
\usepackage{newtxtext}
\usepackage[cmbraces,varbb]{newpxmath}
\usepackage{fix-cm}
\usepackage{lipsum}

\DeclareSymbolFont{lettersCM}{OML}{cmm} {m}{it}
\SetSymbolFont{lettersCM}{bold}{OML}{cmm} {b}{it}
\let\txpi\pi
\DeclareMathSymbol{\pi}{\mathord}{lettersCM}{"19}
\begin{document}
\begin{flushleft}
Pie appears very small
\[\frac{k\pi}{12} = \frac{22}{7}\]
\end{flushleft}

\lipsum[2]

\end{document}

I left flushleft, although it doesn't appear necessary. I changed $$...$$ into the proper \[...\].

Here's the comparison. You will see that the pi is correctly sized and also the lines are well spaced.

The font size is the same in both versions, which have been scaled by the same amount.

Some more information. If I add a command for showing the font used in the document, I get

\T1/ntxtlf/m/n/12=select font ntx-Regular-tlf-t1 at 14.39996pt

for your version with scaling and

\T1/ntxtlf/m/n/14.4=select font ntx-Regular-tlf-t1 at 14.4pt

for my version with extarticle. The difference in the fifth decimal is surely not relevant.

Caveat Don't mix newtxtext with newpxmath: the letters are very different and visually incompatible with each other.