I am looking for some specific welding curtain height requirements.  Everything I've found so far just says that it should accomodate ventilation at ground level and prevent exposure to the arc.  Is there a source out there that gives some guidelines on this subject?

Maybe something in NFPA Standard 51B?
I would think that each application is different and you need to use sound judgement to ensure that the height is sufficient for your application to prevent issues.  

What about spatter??

The height requirement question was specifically prompted by arc radiation issues.  We use only approved tranparent welding curtains in these booths and all combustibles are kept 25' or more away. Also, we weld only aluminum in these local booths, all steel welding is performed inside the maintenance shop with solid walls and a ventilation system.  All current booths were set up by my predecessor and the curtains range anywhere from 12" to 30" from the floor.  I am adding a booth at a sattelite facility and want to make an informed decision on these curtains to make sure we are compliant from the start.   

It might be covered under ANSI Z49.1, but I haven't installed booths for a while. The AWS provides some info (and a free copy of ANSI Z49.1).

http://aws.org/technical/facts/
 

Also check out OSHA 1910.

Your welding curtains must be of sufficient length to prevent incident exposure to the weld arc.  They do not have to be floor to ceiling but should encompass the weld cell.  If you are using robotic welders, my recommendation would be to put welding curtains around the entire perimeter of the robot cell.