Originally Posted by wesmamyke

Well I would say the 25.8. The seat post sizing rods are not conical, they are stepped. So if it did not drop all the way up to the top of the 26.0 it was the one below it. However it is pretty common for the top of the seat tube to be deformed from being tightened too much and you might not want to assume it's the 25.8. Both are pretty uncommon sizes, what brand of frame is it?