could we call this a participle clause? if not what is this sentence?

In the first sentence, the phrase "Just over 8 inches in length" is an adjectival phrase, modifying "cylinder."

Why would you call it a "participle clause"? There's not a participle in it.

There are two kinds of participles- the present participle, which always ends in -ing, and the past participle, which may end in -ed, n or some other ending. It's a form of a verb. There's no verb or form of verb in your phrase, so it can't be a participial phrase. And it's certainly not a claus. A clause has a subject and a verb that carries tense.

"Measuring just over 8 inches in length, the Cyrus Cylinder is made of baked clay." (Now there's a participial phrase- note the -ing on the verb)

"Measured at just over 8 inches in length, the Cyrus Cylinder..." (Now there's a past participle, although this is an unusual construction)

It is AS IF:

The just-over-8-inches-in-length Cyrus Cylinder is made of baked clay."

It's an adjectival phrase.