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.
extarticle
if you want 14pt size. I'm also dubious about the combination ofnewtxtext
andnewpxmath
. – egreg Jan 21 '17 at 16:06newpxfonts
are completely different from and incompatible with the letters innewtxtext
. – egreg Jan 21 '17 at 16:27