Seattle has a 3.25" blade inch limit for carry, but if you are using it on your property that's not considered carry (you can use a machete on your property all day, just don't start roaming the street with it).
Practically speaking if you are hiking with an Opinel no one will care, even more so if it's in your backpack. The exact laws covering legal carry will depend on the jurisdiction(s) you are in while hiking. The state statute on knives doesn't specify a blade length, the issue is how you use it (RCW 9.41.270). However there are certain places you can't carry weapons - jails, courts, liquor sales facilities, and others - however you're not likely to be hiking in any of those (RCW 9.41.300).
Weapons apparently capable of producing bodily harm — Unlawful carrying or handling — Penalty — Exceptions.
(1) It shall be unlawful for any person to carry, exhibit, display, or draw any firearm, dagger, sword, knife or other cutting or stabbing instrument, club, or any other weapon apparently capable of producing bodily harm, in a manner, under circumstances, and at a time and place that either manifests an intent to intimidate another or that warrants alarm for the safety of other persons.
(2) Any person violating the provisions of subsection (1) above shall be guilty of a gross misdemeanor. If any person is convicted of a violation of subsection (1) of this section, the person shall lose his or her concealed pistol license, if any. The court shall send notice of the revocation to the department of licensing, and the city, town, or county which issued the license.
(3) Subsection (1) of this section shall not apply to or affect the following:
(a) Any act committed by a person while in his or her place of abode or fixed place of business;
(b) Any person who by virtue of his or her office or public employment is vested by law with a duty to preserve public safety, maintain public order, or to make arrests for offenses, while in the performance of such duty;
(c) Any person acting for the purpose of protecting himself or herself against the use of presently threatened unlawful force by another, or for the purpose of protecting another against the use of such unlawful force by a third person;
(d) Any person making or assisting in making a lawful arrest for the commission of a felony; or
(e) Any person engaged in military activities sponsored by the federal or state governments.