Answer
$\sqrt{2}
\text{ units}
$
Work Step by Step
Using $|a+bi|=\sqrt{a^2+b^2},$ then the given expression, $
|-1+i|
,$ is equal to
\begin{array}{l}\require{cancel}
\sqrt{(-1)^2+1^2}
\\\\=
\sqrt{1+1}
\\\\=
\sqrt{2}
\text{ units}
.\end{array}