Length is relevant, since it has to do with the gun's actual physical appearance (if anything, it's more relevant than fire modes for anything but semi / burst / auto variants). Weight is kinda iffy but it's part of a general description of the firearm, which is allowed under the rules.
I'd agree things like muzzle velocity, barrel length and so on are going over the top, but the "TMI" guideline is about including pages and pages of material on a gun, not a couple of additional measurements.
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