Marcus says on 1/Mar/10
If you look closely when Thomas is standing next to 5'10" Ralph Waite on "The Waltons", there is at least a 3-4" difference between them. Based on this, Thomas is around 5'6.5" tall.

Mr K says on 29/Mar/05
In the film "Winning" (Paul Newman / Joanne Woodward), Thomas portrays Woodward's characters son. In a line from the film Woodward states to Newman (in relation to Thomas' character), that she has a 5 foot 7 son. I would say that Thomas would have been fully grown (about 17 - 18 years old) when the film was made.

Mr. R says on 28/Mar/05
I used to see Richard Thomas in Hollywood a lot. He used to go to this jazz club called Catalina's. I estimate him at about 5'7" on a good day. His son is pretty tall though, about 6'2", and Richard was always wearing cowboy boots when I saw him to compensate for his lack of height!