How to proper resize blocks to fit text and reducing size of elements such as adder and mixer in Circuitikz?

With pure tikz:

\documentclass[tikz, margin=3mm]{standalone}
\usetikzlibrary{arrows.meta, 
                calc, 
                fit, 
                positioning,
                quotes,
                shapes.geometric}
\usepackage{amsmath}
\tikzset{
alias path picture bounding box/.code=%
    \pgfnodealias{#1}{path picture bounding box},
CNTRL/.style =
{
          > = Triangle,
block/.style = {rectangle, draw, 
    minimum height=8mm, minimum width=16mm,
    outer sep = 0mm},
  dot/.style = {fill,
    circle, inner sep=0mm, outer sep=0mm, minimum size=1mm,
    node contents={}},
 gain/.style = {regular polygon, regular polygon sides=3, shape border rotate=-90, draw,
    inner sep=0pt, anchor=west,
    outer sep = 0mm},
sum/.style = {circle, draw, minimum size=6mm,
    path picture={%
    \tikzset{alias path picture bounding box=@}
    \draw[very thick, shorten <=1mm, shorten >=1mm, -]
    (@.north) edge (@.south)
    (@.west)   --  (@.east);
                },% end of node contents
            node contents={}},
}% end of CNTRL style
    }% end of tikzset

\begin{document}
    \begin{tikzpicture}[CNTRL, node distance = 5mm and 10mm]
\draw[{Circle[open]}->] (0,0) node[above] {$w$} 
                      node[below] {SP} -- ++ (1,0)  node[above left] {$+$}
                                                    node (s1) [sum,right];
\draw[->]   (s1.east) to["$e$"] ++ (1,0) node (g) [gain, right, 
                                                   label={[yshift=-3mm]below:Gain}] {$K_c$};
\draw[->]   (g.corner 1) -- ++ (1,0) node [above left] {$+$}
                                   node (s2) [sum,right];
\node (m1) [block, label={[name=int]below: Integral},
            below right=of s2] {$\dfrac{1}{T_i s +1}$};
\node (d1) [dot, right=of s2 -| m1.east]; 
\draw (s2) -- (d1); 
\draw (d1) to["$u$", "MW" '] ++ (1,0) node (m2) [block,right] {$\dfrac{k_p}{a s^2+b s+1}$};
\draw (m2.east) -- ++ (1,0) node (d2) [dot];
\draw[-{Circle[open]}]  (d2) -- ++ (1,0);
% inner loop
\draw[->]   (d1) |- (m1);
\draw[->]   (m1) -| (s2.south)  node[pos=0.25, above] {$u_f$}
                                node[below left] {$+$};
% outer loop
\coordinate[below=of int] (aux);
\draw[->]   (d2) |- (aux) -| (s1.south) node[below left] {$-$};
% fit
\node[draw=teal, dashed, inner sep=2mm, yshift=2mm, fit=(g) (d1) (int)] {};
    \end{tikzpicture}
\end{document}

You can always restructure and scale things to fit ...

\documentclass[varwidth,border=3mm]{standalone}
\usepackage{circuitikz}
\begin{document}
\begin{circuitikz}
\ctikzset{blocks/scale=0.5}
\draw (0,0) node[adder] (m) {} 
(m.1) to[short,-o] ++(-1,0) node[](sp){}; 
 \draw (m.2) node[below left] {\scriptsize $-$};
 \draw (m.1) node[above left] {\scriptsize $+$};
  \draw(m.1) node[inputarrow] {}; 

\ctikzset{ blocks/scale = 1}
\draw (m.3) to[short,>] ++(0.5,0) to[amp, t=$K_c$,l_={\scriptsize Gain},>] ++(1.25,0) node[] (p){};
\ctikzset{ blocks/scale = 0.5}

\draw  (p) to[short,>] node[adder, fill=white] (int){} ++(0.75,0) (int.1) node[inputarrow] {};
\ctikzset{ blocks/scale = 1}

\draw (int.3) to[short] ++(2,0) node(ua) {} to[short] ++(0.25,0) to[twoport, l_={\scriptsize Process}, 
t=\scalebox{0.5}{$k_p/(\begin{array}[t]{@{}c@{}}a s^2\\
+b s\\
+1)\end{array}$},> ] ++(2.25,0) node(pv) {} to[short] ++(0,-2.25)  node (a){} to[short] (a -| m.2) to[short,>] (m.2)node[inputarrow,rotate=90]{};
\draw (pv) to[short,*-o] ++(0.5,0) node[] (pv2) {};

\draw (ua) to[short,*-] ++(0,-1) node[] (ua2){} to[twoport,l={\scriptsize Integral}, t=$\frac{1}{T_i s+1}$,>] ++(-2,0) node (uf){} to[short] (ua2 -| int.2) to[short,>] (int.2)node[inputarrow,rotate=90]{};
 \draw (int.2) node[below left] {\scriptsize $+$};
 \draw (int.1) node[above left] {\scriptsize $+$};
 \draw (ua)  node[above right]() {\scriptsize \quad $u$} (ua)  node[below right]() {\scriptsize\quad  MV};
 \draw (sp)  node[above](){\scriptsize $w$}  (sp)  node[below](){\scriptsize SP};
 \draw (pv2) node[above](){\scriptsize $y$}  (pv2) node[below](){\scriptsize PV};
 \draw (m.3) node[above right](){\scriptsize $e$};
 \draw (uf) node[above](){\scriptsize $u_f$}  (pv2);

\draw[dashed, cyan!70!black] (0.55,-2) rectangle ++(4.755,2.75);
\end{circuitikz}
\end{document}

... but there is probably very little doubt that this is not pretty. So IMHO the real question is whether you really need to put such lengthy expressions in the elements of the circuit. Wouldn't it be better if you just put some well readable letters and use the text to explain what they mean?