The XT Tamper has an adjustment range of approx. 20mm - since these are cast parts there is always some variance but 20mm give or take is pretty good.  The smallest it can be is around 80mm high and the largest is around 100mm high.  

Adjustment is dead easy thanks to the patent pending hidden stepless adjustment mechanism invented by Paul Pratt.  There are no ugly spacers or washers to deal with and you can set the height at any point along the adjustment range.

So to adjust:

Unscrew the main handle by rotating anti-clockwise.

Insert the allen key (supplied with the tamper) and turn the adjustment screw.

Clockwise = shorter tamper

Anti-clockwise = taller tamper

After that you fasten the main handle back on by screwing it clockwise and firmly lock it down.  It may take a bit of trial and error to work out the height you need but it is fairly easy.  

Do not make the handle too tall so that the silicone o-ring is visible.  Also don't make it so short that you force the handle onto the flared section of the stem.