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