the table or the tree?

the tree was 18 feet long...

Probab;ly the LOG , from which the table was made, was 18 feet long.

It evidently is intended to mean the table, although the sentence is badly punctuated and logically (and grammatically) it means the tree.

To make the meaning clear, the original sentence needed another comma, thus:

...a custom-made dining table, cut from a single Claro Walnut tree from Oregon, that's 18 feet long.

"Furnishings include a custom-made dining table, cut from a single Claro Walnut tree from Oregon that's 18 feet long."

"That's" is a contraction of that is.Something in the present is 18 feet long. It can't be the tree, which no longer exists. Anyhow, Tables have length - trees have height - a tree would be 18 feet tall.

To have gotten a dining table from one tree, it would have had to be a tree/log that was much longer than 18 feet.

Furnishings include a custom-made dining table, cut from a single Claro Walnut tree from Oregon that's 18 feet long.

The grammar tells us that it can be nothing but the table.

... a custom-made dining table. It's 18 feet long. [It was] cut from a single Claro Walnut tree from Oregon.

Furnishings include a custom-made dining table, [which was] cut from a single Claro Walnut tree from Oregon.

Whichever it is, I don't want a table that long in my dining room. Wouldn't fit anyway.

once the tree is cut and on the ground it has length, no?

OK then.