The OE riser is 3 1/8" tall. You can go to a 4" or perhaps a little more without changing anything. Because they sit on an angle on the triple tree (and are mostly curved, except for riser spacers), measurements are a bit tricky.
These are a bit steep, but they are 4" (or so), and give about as much rise and pullback as you can get without changing anything. (except you might need to turn the wire clips on the back of the handlebars around 180*, and that is easy peasy)
http://www.aeromachmfg.com/RISER_SET_4_VTX_Retro_1300_p/am-4120c.htmThese are cheaper ones, that one of our members had crack clear through on one side at the bolt holding the bar.
http://www.jpcycles.com/product/ZZ37171These are riser spacers (you just add to your existing risers), and cheaper yet. And if they add an inch to 3 1/8" stock risers, then total height should be 4 1/8" (but they may not give quite as much pullback as a complete riser set, but I don't know).
http://www.twistedthrottle.com/handlebar-risers-honda-shadow-valkyrie-magna-vtx1300-pacific-coast-pc800-with-1-inch-diameter-handlebarsYou can also look at a site called
Scootworks. http://www.scootworks.com/shop.cgi/page=partshonhandle.htm/SID=PUT_SID_HERE
