Well, it depends. If your talking about a '48 Harley big twin frame and your using stock type ball bearing neck cups, they measure about 7 5/16" from the top of the upper race to the bottom of the bottom race when every thing is assembled. With a Harley springer you would add the height of the pre-load nut, the thickness of the top tree, plus the top nut to that figure. So. . . in addition to that 7 5/16", it will depend on your top tree, how you apply your bearing pre-load (you can use a thin nut if neccesary), and the thickness of the nut that retains and centers your top tree. Pictures and more info would probably get you a more accurate answer.
With ball bearing neck cups and a stock frame it measures about 6 13/16" from top of top cup to bottom of bottom cup.