"Noi măsurăm lungimea cu o riglă."

Translation: We measure the length with a ruler.

[ZsuzsiHUN]

I think the English translation is wrong, you do not use "the" in this case. It should be: "We measure length with a ruler."

[ArguingInElvish]

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.

[Miki8307]

You all are right here. "Lungimea" means "length" when it refers to length in general, and it means "the length" when it refers to the length of a specific object. Without a context, my first choice would be "length" without "the".

[Howard]

"We measure the length..." of something specific.
"We measure length..." in general.