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)

The reason we say, for example, "5 metres in length" is to show that the measurement of 5 metres is the length and not any other dimension such as width. 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 5-metre minimum lengths.*

_{*you have to be careful this does not sound like you mean 5 lengths of one metre each, because a 1-metre length can be called "a metre length".}

or

We can cut timber into lengths no shorter than 5 metres.

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

or

The minimum length we can cut timber into is 5 metres.

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