"As Greek scholars—even non-Catholic ones—admit, the words petros and petra were synonyms in first century Greek. They meant “small stone” and “large rock” in some ancient Greek poetry, centuries before the time of Christ, but that distinction had disappeared from the language by the time Matthew’s Gospel “was rendered in Greek. The difference in meaning can only be found in Attic Greek, but the New Testament was written in Koine Greek—an entirely different dialect. In Koine Greek, both petros and petra simply meant “rock.” If Jesus had wanted to call Simon a small stone, the Greek lithos would have been used.”
catholic.com/tracts/peter-the-rock
"Greek is an inflected (not “reflexive”) language, which means that the forms of nouns change based on the function a word is performing in a sentence. When this happens, the base meaning of the word remains the same. The inflection communicates information about how the word is being used grammatically but not what it means.
In the case of petros vs. petra, the change is not an inflection. Petros and petra are two different words in Greek. They are similar because they are cognates (just as “president” and “presider” are cognates in English but are nonetheless two different words with different, though related, meanings). Because they are two different words, the inflection (change of form) of petros and petra is not what is at issue here. The basic meanings of the terms is.
The point the article is making is that in Attic Greek there was a slight difference in meaning between the two, but in Koine Greek (the dialect of the New Testament) they were synonyms. A place to look this up is D. A. Carson’s commentary on Matthew 16 in the Expositors Bible Commentary. He makes this point very well, and he is a highly-respected Evangelical Bible scholar. "
jimmyakin.com/2004/04/petros_vs_petra.html
“you are Peter, and on this rock I will build my church. This is one of the most controversial and debated passages in all of Scripture. Roman Catholics have appealed to this passage to defend the idea that Peter was the first pope. The key question concerns Peter’s relationship to “this rock.” In Greek, “Peter” is Petros (“stone”), which is related to petra (“rock”). **The other NT name of Peter, Cephas (cf. John 1:42; 1 Cor. 1:12), is the Aramaic equivalent: kepha’ means “rock,” and translates in Greek as Kēphas. “This rock” **has been variously interpreted as referring to (1) Peter himself; (2) Peter’s confession; or (3) Christ and his teachings. For several reasons, the first option is the strongest. Jesus’ entire pronouncement is directed toward Peter, and the connecting word “and” (Gk. kai) most naturally identifies the rock with Peter himself.”
- Protestant ESV bible commentary