While it is extremely unlikely that the inspecting Coast Guard officer is going to get out a tape measure and verify the length of a commercially produced boat, the definition of "length" is not arbitrary but is spelled out in clearly in the Code of Federal Regulations.
Railings, outboards, anchor rollers, etc do not add to the length of the vessel. USCG personnel are professional and know the rules.
Code of Federal Regulations Title 46, Chapter 1, Subchapter M, Part 136, Subpart A, Section 136.110 (Definitions)
Length means the horizontal distance measured from end to end over the deck, excluding the sheer. Fittings and attachments are not included in the length measurement.
I have less faith in the knowledge of the average deputy, game warden, or other normally land-based law enforcement officer. They may or may not be as well-versed in the minutiae of the regulations. With them it is probably best to fall back on your state registration which
should plainly document that your boat is 25' 10".
Now if you want to talk about marinas or harbor masters and how they measure boats for docking purposes, that is another story all together. You are most certainly 26' and maybe even longer depending on what you have bolted onto your deck (I'm looking at you Highlander)

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