long lengths of adjustable shelves

Question

What is the singular of this phrase?

long lengths of adjustable shelves

Is the singular number of long lengths of adjustable shelves this?

an adjustable shelf with long lengths

Please correct it.

Answer 1

A length can be used as a countable measure (albeit not clearly defined), as shown by Macmillan:

3 [COUNTABLE] a piece of something that is long and thin
a length of pipe/rope/string

But in that case we usually do not add an adjective to it like long. It can be used in a way similar to meter:

I bought three lengths of the finest silk.

Obviously, in the singular that would be:

I bought a length of the finest silk.

Adding long to it can be done to emphasize the extent of the lengths, as we can do with kilometers:

He bought 100 long lengths of cloth.
He walked 10 long kilometers home.

The actual size of a lengths or a kilometer is not longer than usual, but is may appear longer.

When using long with a singular length, I can only assume that is is longer than one would expect (which is easy, since I do not think a length is a very well-defined measure of size).

So the singular of your sentence would be:

a long length of adjustable shelf.

Note that I also changed shelves to shelf, although that doesn't make it really singular. I would actually have used shelf in your original sentence, using it as a mass-noun, or alternatively, shelving, as Andrew Leach noted.