I would say "that length nails," but I doubt I can explain why. " My instinct is that "length" is a noun here, and is singular, as there's only one length. Of course "that length" is a modifier, but I don't know how you can make it "agree" with "nails" in the plural.
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Anonymousexcuse me! but I think the correct way to say is "those length nails"You're excused.
AnonymousI would get those length nails but the ones are too long for my taste.I previously decided to try to answer your question and let the other errors in the sentence slide, but I don't want to get skunked.