Does the Bible call all unrighteousness sin, even small sins?

Scripture states that all unrighteousness is sin, and it makes no distinction by size: whether it is a little sin or a great one, it is still sin. The same passage also notes that there is a sin not unto death, implying some sins are more severe.

Les teaches that all unrighteousness is sin, whether it be a little sin or a great one—it is still sin. He then notes there is a sin not unto death, and asks what is implied: some sin is gross enough that God will take that believer out ahead of time. To illustrate, Les points to 1 Corinthians chapter 5, where a man is evidently committing a sin unto death. It is worthy of death if he does not repent and turn around. If he continues living with his stepmother in a marital relationship, God will take him out of Corinth. Verse 5 says to deliver such a one unto Satan for the destruction of the flesh, that the spirit, that is the soul of the man, may be saved in the day of the Lord Jesus. Then Paul comes back to the Corinthians as a congregation: they were glorying and puffed up, and their glorying was not good. Know ye not that a little leaven leaveneth the whole lump? Les applies this to the church of today, recalling a man who said the church got into dire straits because as various social sins came along, it said it could accommodate them and live with them, then came something more drastic, and it still did not want to cause a ruckus.

1 John 5:17

All unrighteousness is sin: and there is a sin not unto death.

Is all unrighteousness sin, even small sins?

Yes. Whether a sin is little or great, it is still sin, and the text calls all unrighteousness sin. It also distinguishes a sin not unto death from sin gross enough that God may take a believer out ahead of time. The size of the sin does not change its moral character as sin.