The SCS Ionograph SMD Module II has a version that appears to be capable of accepting large boards, up to 26"x38".  

http://www.scscoatings.com/docs/SCS-Ionic-Contamination-Test-Systems.pdf

Range of Test Cell sizes are published at:

18 x 20 in /  20 x 26 in /  26 x 30 in /  26 x 38 in / 

Shean Dalton