The reason we say, for example, "5 metres *in length*" is to show that the measurement of 5 meters is the length and not any other dimension such as breadth. We don't always prefix "length" with "in".

You are right in saying that "lumber" and "timber" are non-countable nouns, but "*a length*" can be used as a noun for a specific cut of timber, the plural of which is "*lengths*".

So you could say:

> We can cut timber into a minimum of 5 meters *lengths*.

or

> We can cut timber into lengths no shorter than 5 meters.

(these are using "length" as a noun for the piece of cut timber)

**or**

> The minimum length we can cut timber into is 5 meters.

(this is using "length" as a noun for the measurement)