I will post textual evidence supporting the Witherington hypothesis that the anonymous author of Hebrews was influenced by Galatians; however I will not be limiting myself to Witherington’s interpretation of the texts.  I will begin where Witherington begins in his 1991 NTS article: “πηλίκος”, “how large” (referring to size) or “how great” (referring to dignity). 

It seems to me that, in all usages, there is a play on the two senses of the word: size and greatness.

A rare word, it appears in the LXX only once. Zechariah 2:1-4, concerning size, or dignity/glory? It is pretty absurd to measure angelic fire with a measuring line!

“Then I lifted up my eyes and saw a man with a measuring line in his hand. Where are you going?” I asked. “To measure Jerusalem,” he replied, “and to see the size of its width (τοῦ ἰδεῖν πηλίκον τὸ πλάτος αὐτῆς ἐστιν) and the size of its length (καὶ πηλίκον τὸ μῆκος).” Then the angel who was speaking with me went out, and another angel came out to meet him and said to him, “Run and tell that young man: ‘Jerusalem will be a city without walls because of the multitude of men and livestock within it. For I will be a wall of fire around it, declares the LORD, and I will be the glory within it.’ 

It appears twice in the NT.

Galatians 6:11, used literally in reference to size, but is that all?

“See how large letters I have written to you in my own hand.”

Ἴδετε πηλίκοις ὑμῖν γράμμασιν ἔγραψα τῇ ἐμῇ χειρί.

There seems to be a size comparison between the letters written by Paul, and those in the body of the letter dictated to a professional. The letters are presumably larger, although they may also have been smaller. Either way, the size of the letters is an odd thing to focus on. There may be an implicit comparison between the humility of the signature letters in his hand and the dignity of the stigmata of Jesus–his real signature–in his body: “I bear the marks of Jesus in my body” (6:17).

Hebrews 7, πηλίκος refers to the dignity of Melchizedek. But this claim is based on a small number of OT texts: Just, Gen. 14:18-20, and Ps.110:4). And in the context of the homily, the real measure greatness is Jesus who guarantees a “better covenant”:

“He met Abraham returning from the slaughter of the kings and blessed him, and Abraham apportioned to him a tenth of everything….Consider how great this one was [Θεωρεῖτε δὲ πηλίκος οὗτος, i.e. Melchizedek]… the lesser [Abraham] is blessed by the greater [Melchizedek]… This becomes even more evident when another priest arises in the likeness of Melchizedek (κατὰ τὴν ὁμοιότητα Μελχισέδεκ)…but Jesus became a priest with an oath by the One who said to Him: “The Lord has sworn and will not change His mind: ‘You are a priest forever.’ ”Because of this oath, Jesus has become the guarantee of a better covenant.”

Could it be that Hebrews not only used a Paul’s word but also echoed the connection between the humble one, Paul, his hand, his body, and the one worthy of ultimate esteem: Jesus. As “great” as Melchizedek may be, Jesus offers something better.

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The play on large vs small, and relative dignity can be seen in the use of a closely related NT word used by James, Ch. 3:

“Not many of you should become teachers, my brothers, because you know that we who teach will be judged more strictly. We all stumble in many ways. If anyone is never at fault in what he says, he is a perfect man, able to control his whole body.

When we put bits into the mouths of horses to make them obey us, we can guide the whole animal. Consider ships as well. Although they are so large (τηλικαῦτα) and are driven by strong winds, they are steered by a very small rudder wherever the pilot is inclined.

In the same way, the tongue is a small part of the body, but it boasts of great things. Consider the size [ἡλίκον] of a fire, the size [ἡλίκην] of a forest ablaze. The tongue also is a fire, a world of wickedness among the parts of the body. It pollutes the whole person, sets the course of his life on fire, and is itself set on fire by hell.”

I wonder if, like the author of Hebrews, the author of the letter attributed to James was interacting with Galatians. Did Paul imply a comparison of the humble size of the letters, and the the stigmata of Jesus in his body! The reader of James will ask: Does he have perfect control over his body? Or over his small tongue (that dictates letters?) If Galatians is the rudder of the ship of the church, where will it lead when winds are strong?