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.
Les explains that in verse 17 all unrighteousness is sin, whether it be a little sin or a great one; it is still sin. The text then says there is a sin not unto death, implying that some sin is gross enough that God will take that believer out ahead of time. Les connects this to 1 Corinthians chapter 5, where a man is evidently committing a sin unto death. The instruction is to deliver such a one unto Satan for the destruction of the flesh, that the spirit, the soul of the man, may be saved in the day of the Lord Jesus. The man is worthy of death if he does not repent; if he continues living with his stepmother in a marital relationship, God will take him out of Corinth. Les also notes the congregation was glorying and puffed up, and that a little leaven leaveneth the whole lump. He applies this to the church of today, recalling a message that the church got into dire straits because it accommodated social sins, then more drastic ones, to avoid a ruckus.
All unrighteousness is sin: and there is a sin not unto death.