Correct format for [Plural noun] of [physical quantity] [values]

That's a very awkward construction, "of length." You would say, "Two lines, 9 cm and 10 cm long,..." which actually isn't a sentence. There's no verb in there.

As a math teacher, I'd say, "Please draw two lines, one 9 cm long and one 10 cm long." As Barn and Phil noted, "of length" doesn't sound right.

The plural would be correct. "We have two materials of dielectric constants 7 and 8." The nouns should agree with the plural verb "have". So you "have materials...of constants...".

And the original sentence> "The transistors, 10 cm and 9 cm long, have demonstrated a linear change of 50% and 76%, respectively."

Hope that helps.