\def\a{3}
\def\b{3}
\def\r{2cm}

\foreach \i in {1,...,\a} 
    \node[draw, small vertex] (a-\i) at (\i*\r, \r) {};
    
\foreach \i in {1,...,\b} 
    \node[draw, small vertex] (b-\i) at (\i*\r, 0) {};

\foreach \u in {1,...,\a}
    \foreach \v in {1,...,\b}
        \draw (a-\u) -- (b-\v);