what should be the hydraullic dia considered for finding the boundary layer parameters for flow through venturimeter Should it be pipe dia or throat dia ?
If you use the throat, that will give you the 'worst case' for first layer height. That will be the area of highest velocity, so smaller first layer height for same Y+. The pipe diameter will be larger so that same first layer height will yield a smaller Y+.
You may be better off using different first layer heights for the different regions - or setting the first layer height as a function of surface mesh size. That way where surface mesh is smaller, the first layer height will also be smaller.
