Custom font size in a math aligned environment

Question

I am fairly new to LaTeX and I need help in finding a way to increase the font size in a  math aligned environment.
documentclass[10pt]{article}
usepackage{amsmath}
usepackage{relsize}

newcommand{eq}[1]{begin{align*}
    #1
    end{align*}}

begin{document}

eq{
fontsize{20}{20}
    &F=ky
    &W = frac{1}{2}ky^2
}

end{document}

I have tried using the large and the fontsize{}{} command and it has not worked. I would be greatful if someone could help me out in ajusting the font size of my equations. Ideally I would like a solution where I can enter another parameter into my eq command to control the font size.

Ivan · Accepted Answer

This is an unconventional solution which uses the fontsize package. We define the new environment myalign* with an optional argument which takes one of the font sizing commands provided by fontsize. This should be sufficient for most concrete cases.
documentclass[10pt]{article}
usepackage{lmodern}
usepackage{amsmath}
usepackage[fontsize=10pt]{fontsize}

makeatletter
newenvironment{myalign*}[1][]{%
 csname#1endcsname
 start@align@nest@rredtruem@ne
}{%
 endalign
}
makeatother

begin{document}

Normal size

begin{myalign*}
 &F=ky
 &W = frac{1}{2}ky^2
end{myalign*}

begin{myalign*}[normalsizer]
&F=ky
&W = frac{1}{2}ky^2
end{myalign*}

begin{myalign*}[normalsizerr]
 &F=ky
 &W = frac{1}{2}ky^2
end{myalign*}

begin{myalign*}[normalsizerrr]
 &F=ky
 &W = frac{1}{2}ky^2
end{myalign*}

begin{myalign*}[Larger]
 &F=ky
 &W = frac{1}{2}ky^2
end{myalign*}
 
 Normal size
 
end{document}

If you want full control over the font and the line spacing you will need something similar:
makeatletter
newenvironment{xmyalign*}[2]{%
 fontsize{#1}{#2}selectfont
 start@align@nest@rredtruem@ne
}{%
 endalign
}
makeatother

To use, for example, like this:
begin{xmyalign*}{60}{60}
 &F=ky
 &W = frac{1}{2}ky^2
end{xmyalign*}

Custom font size in a math aligned environment

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