I think the English translation is wrong, you do not use "the" in this case. It should be: "We measure length with a ruler."
"We measure the length..." of something specific. "We measure length..." in general.
As for how one would say it in English, I think both work. Omitting "the" speaks to a generalization of length and including "the" speaks to a particular length.
As for this exact translation from Romanian "lungimea" = "the length" (lungime = length; the 'a' adds 'the')
I am still learning myself so feel free to correct me; thanks for the help.