How to increase the size of a boson in tikz-feynman package?

It is possible, as described in the manual, to create your own style. Here I have created the yourboson style colored blue (but of course modifiable).

tikzfeynmanset{
yourboson/.style={
blue,
decoration={zigzag,amplitude=2mm,segment length=4mm,pre,pre length=2pt, post,post length=2pt},decorate}
}

The advantage is that it keeps the original style intact.

documentclass[border=5mm]{standalone}
usepackage[compat=1.1.0]{tikz-feynman}

begin{document}
usetikzlibrary {decorations}

tikzfeynmanset{
yourboson/.style={
blue,
decoration={zigzag,amplitude=2mm,segment length=4mm,pre,pre length=2pt, post,post length=2pt},decorate}
}

begin{tikzpicture}
  begin{feynman}
    vertex (a);
    vertex [above right = 5 cm of a] (u11);
    vertex [above left = 5 cm of u11] (u12);
    vertex [right = 0.4 cm of a] (b);
    vertex [above right = 5 cm of b] (u21);
    vertex [above left = 5 cm of u21] (u22);
    vertex [right = 0.4 cm of b] (c);
    vertex [above right = 5 cm of c] (u31);
    vertex [above left = 5 cm of u31] (u32);
    vertex [above right = 2 cm of u31] (f1);
    vertex [above right = 3 cm of f1] (f2);
    vertex [below right = 3 cm of f1] (f3);
    
    diagram* {
      (a) -- [fermion] (u11) -- [fermion] (u12),
      (b) -- [fermion] (u21) -- [fermion] (u22),
      (c) -- [fermion] (u31) -- [fermion] (u32),
      (u31) -- [yourboson] (f1),
      (f1) -- [fermion] (f2);
      (f1) -- [fermion] (f3);
    };
  end{feynman}
  
end{tikzpicture}

end{document}

Asymptote can do it.

Compile here http://asymptote.ualberta.ca/

The Feynman diagram for β− decay.

usepackage("amssymb");

path zigzag(path g, int n=5, real amplitude=1 , real tension1=100, real tension2=tension1){

// n : the number of segments of path g (the default value is 5)
// amplitude : the amplitude of zigzag (the default value is 1)
// To understand tension, see https://asymptote.sourceforge.io/doc/Bezier-curves.html#Bezier-curves

pair[] A;
for (int k=0; k <= n; ++k){ A.push(point(g,k/n)); }
pair[] midofA;
pair[] vertexofcurve;
for (int i=0; i < A.length-1; ++i){ 
  midofA.push(0.5*(A[i]+A[i+1]));
  vertexofcurve.push(midofA[i]+amplitude*unit(rotate((i%2==0) ? 90 : -90)*(A[i+1]-midofA[i])));  
}
guide zz=A[0];
for (int i=0; i < vertexofcurve.length; ++i){
zz=zz.. tension tension1 and tension2 ..vertexofcurve[i]
     .. tension tension2 and tension1 ..A[i+1];
}
return zz;
}
// An example :
/*
unitsize(1cm);
path g=(0,0)--(5,2);
draw(g);
draw(zigzag(g,amplitude=1.5,tension1=10,tension2=4),red);
draw(zigzag(g,amplitude=0.8,tension1=10,tension2=1),green);
draw(zigzag(g,n=18,amplitude=0.2),blue);
*/
// we will get the zigzag if tension1 is big.

unitsize(1cm);
path q1=(0,0)--(2,3),q2=(2,3)--(0.1,6);
transform t=shift(0.3,0);

draw(shift(0,-3)*Label("u",BeginPoint+(0,-0.2)),q1,MidArrow(size=5));
draw(Label("u",EndPoint),q2,MidArrow(size=5));
draw(Label("d",BeginPoint+(0,-0.2)),t*q1,MidArrow(size=5));
draw(Label("d",EndPoint),t*q2,MidArrow(size=5));
draw(Label("d",BeginPoint+(0,-0.2)),t^2*q1,linewidth(1bp),MidArrow(size=8));
draw(Label("u",EndPoint),t^2*q2,linewidth(1bp),MidArrow(size=8));
path ZigZag=zigzag(t^2*(2,3)--t^2*(2,3)+dir(20),
                   n=4,
                   amplitude=0.28,
                   tension1=5,tension2=2.2);

draw(Label("$W^{-}$",Relative(.9),2*S),ZigZag,linewidth(1bp));
draw(Label("$e^{-}$",EndPoint),
     relpoint(ZigZag,1)--relpoint(ZigZag,1)+1.2*dir(45),
     linewidth(1bp),MidArrow(size=8)); 
draw(Label("$overline{mathcal{V}}_{e}$",BeginPoint),
     reverse(relpoint(ZigZag,1)--relpoint(ZigZag,1)+1.4*dir(80)),
     linewidth(1bp),MidArrow(size=8)); 

draw(scale(1.5)*Label("$t$",EndPoint),(-.5,-.5)--(-.5,6.5),Arrow);
label(scale(1.5)*"$n$",(0.2,-.7));
label(scale(1.5)*"$p$",(0.3,6.7));

The picture of example:

And here is diagram (a) (your link)

path zigzag(path g, int n=5, real amplitude=1 , real tension1=100, real tension2=tension1){

// n : the number of segments of path g (the default value is 5)
// amplitude : the amplitude of zigzag (the default value is 1)
// To understand tension, see https://asymptote.sourceforge.io/doc/Bezier-curves.html#Bezier-curves

pair[] A;
for (int k=0; k <= n; ++k){ A.push(point(g,k/n)); }
pair[] midofA;
pair[] vertexofcurve;
for (int i=0; i < A.length-1; ++i){ 
  midofA.push(0.5*(A[i]+A[i+1]));
  vertexofcurve.push(midofA[i]+amplitude*unit(rotate((i%2==0) ? 90 : -90)*(A[i+1]-midofA[i])));  
}
guide zz=A[0];
for (int i=0; i < vertexofcurve.length; ++i){
zz=zz.. tension tension1 and tension2 ..vertexofcurve[i]
     .. tension tension2 and tension1 ..A[i+1];
}
return zz;
}

unitsize(1cm);
transform t=shift(0,-4);
draw(Label("$pi$",BeginPoint),(0,0)--(2,0),MidArrow);
draw(Label("$pi$",EndPoint),(2,0)--(4,0),MidArrow);
draw(zigzag((2,0)--t*(2,0),n=10,amplitude=0.5,tension1=100));
draw(Label("$N$",BeginPoint),t*((0,0)--(2,0)),MidArrow);
draw(Label("$N$",EndPoint),t*((2,0)--(4,0)),MidArrow);
dot((2,0)^^t*(2,0));

And you can draw your Feynman diagrams.