7 speed freehubs measure 32mm from the raised (taller)spline outer edge, to the outer edge of the freehub body(where the cassette retainer ring screws into the hub)... Measure along the top of the spline, not in the valley between the splines.

8-9-10 speed freehubs measure 35mm

these measurements are from the portion of the splines that are machined to a larger diameter, Not from the inner flange area of the freehub.
http://www.bikeplus.co.uk/images/pro...dy-W211339.jpg

SOME freehubs are made with no increase in spline diameter, measure it from the inner flange, to the outer edge of the freehub, along the top of one of the splines..... this design is fairly rare... some have very little "spline relief"... once again, measure along the top of the spline, not in the valley between the splines, ok?

IF the freehub body has EXTERNAL threads, it is an old style freehub known as "uniglide" they are all 7 speed length, as far as i've seen....