\documentclass{article}
\usepackage[paperheight=50mm,paperwidth=95mm, margin=5mm]{geometry}
\usepackage{feynmp-auto}
% commas in feynmp sometimes lead to strange errors, use \comma instead
\DeclareMathSymbol{\comma}{\mathpunct}{letters}{"3B}

\begin{document}\LARGE
    \centering
    \begin{fmffile}{bsll_leptoquark_diagram} \centering 
        \begin{fmfgraph*}(250,100) 
            \fmfstraight               
            \fmfleft{l1,l2}
            \fmfright{r1,r2}     
                          
            \fmf{phantom}{l1,p1,r1}
            \fmf{phantom}{l2,m1,m2,m3,r2}            
            
            \fmffreeze
            
            \fmf{fermion,label=$b$,tension=1.0}{l2,m1}       
            \fmf{photon,label=$\phi$,tension=1}{m1,m3}    
            \fmf{fermion,label=$s$,tension=1.0}{m3,r2}
            
            \fmf{fermion,label=$\l^-$, label.side=right}{m1,p1}
            \fmf{fermion,label=$\l^+$}{r1,m3}
            
        \end{fmfgraph*}
    \end{fmffile}
    \pagestyle{empty}
\end{document}