documentclass[11pt]{standalone}
usepackage{tikz}

begin{document}
begin{tikzpicture}


coordinate[label=left:A]  (A) at (0,0);
coordinate[label=right:B] (B) at (3,0);

newcommand{myLabel}{$sin(x)+cos(y)=int_0^infty e^{x y}, dx$}

draw (A) --  node[above] {myLabel}  ++(B);

end{tikzpicture}
end{document}

newcommand{myLabel}{{footnotesize{$sin(x)+cos(y)=int_0^infty e^{x y}, dx$}}};

newcommand{myLabel}{{tiny{$sin(x)+cos(y)=int_0^infty e^{x y}, dx$}}};