#JavaScript (ES8), 43 41 39 26 bytes
n=>`_=>"".padEnd(${n}*19)`
##Test it
The output of the generated function is a string of spaces, which have been replaced in the Snippet with *s so they can be seen.
g=
n=>`_=>"".padEnd(${n}*19)`
o.innerText=(h=n=>`Function: ${x=g(n)}\nLength: ${x.length}\nOutput: ${x=eval(x)().replace(/./g,"*")}\nLength: `+x.length)(i.value=10);oninput=_=>o.innerText=h(+i.value)<input id=i type=number><pre id=o>