WASHINGTON — Zach Thornton’s relatively slender frame for a pitcher might benefit him.

Listed at 6-foot-3, 170 pounds, the left-hander — who will pitch for the Mets on Wednesday in his major league debut (either as the starter or behind an opener) — credits that build for his approach on the mound.

“I’m not the biggest guy in the world,” Thornton said Tuesday at Nationals Park.

“Everything you do is for the people who said, ‘He couldn’t do it.’ And the people who said, ‘He could.’ ”

Thornton, who has endeared himself to Mets officials in part because of his ability to throw strikes, is receiving the first shot at filling the vacancy created by Clay Holmes’ fractured right fibula. Holmes was injured Friday when the Yankees’ Spencer Jones hit a line drive off his leg.

In seven minor league starts this season he has pitched to a 3.16 ERA. Thornton appeared twice for the team in the Grapefruit League and left a positive impression on manager Carlos Mendoza.

“I treat every game the same: I want to go out and compete,” Thornton said. “I am the ultimate competitor, a strike thrower.”

Thornton was selected over Jack Wenninger and Jonah Tong at Syracuse. The Mets also have potential starting options in the bullpen in Tobias Myers and Sean Manaea.

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Mendoza was asked if Nick Morabito’s promotion to the major leagues (joining fellow rookies Carson Benge and A.J. Ewing in the outfield) means Juan Soto will be relegated mostly to DH.

“[Soto] is our left fielder, but hopefully we get some of the other guys from the injury list and those guys are going to need some DH appearances and Juan is going to be playing a lot of left field,” Mendoza said. “So right now while we can we will just use those [DH for Soto] days as much as possible.”

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Entering play the Mets had participated in 10 extra-inning games this season, the most in the major leagues.

They had won six of those games, also the most in MLB. The most recent was their 16-7 victory over the Nationals in 12 innings Monday.

The Mets set a franchise record with 10 runs in an extra-inning frame. It was tied for fourth highest in major league history.