Swapping cranks, but the new crank is wider, how do I compensate for this? I was swapping the cranks and bottom bracket on my karate monkey from a Truvative stylo crankset (Mountain Double, GXP bottom bracket) to a Race Face Evolution DH (Mountain Triple, Race Face bottom bracket, It's Shimano Hollowtech II compatible I think).

However, the width (Q-factor?) of the Race Face triple crankset is wider than the Truvative Stylo.  If I install it and tighten the bolt, it looks to be 5mm+ of exposed space on the crank spindle and the crank arms can slide side to side through the bottom bracket since the spindle is so long.  I'm pretty sure my BB shell is 73mm (according to surly).

What do I need to do to compensate for this space?  Spacers between BB Shell and BB Cup? How many spacers will I need, and of what size? Is there some common spec listed for the cranks that I should have been able to tell this from the start?
You're right, you want to space it between the shell and cup.  You can put spacers on either side to get the chainline right.  Most cranksets come with spacers (2.5 mm is probably the most useful size for you) but if yours didn't any LBS should have a few to sell you.  Something like this: http://wheelsmfg.com/bottom-bracket-spacer.html (I don't know if these work with hollowtech BB's or not)

You'll probably want to get an assortment of sizes if you can, and use as few as possible to get things lined up (I assume this would help maximize BB stiffness, but really I just hate having excessive spacers - makes it more likely I'll lose one when disassembling/reassembling).