I know this topic has been covered before but it's worth coming back to.
Say you want your avatar to be 5ft 6in tall (1.68m) tall. Using the standard Linden Lab viewer, if you set that height on the shape slider, you'll be considerably taller than a prim of the same height. In other words, there's a mismatch between the scale of the appearance slider and that of of prims.
I use the Alchemy viewer, where the shape slider value has been amended to correspond very closely to prim scale. T