Screen grabs from the various DVD's that are out there will be your best bet. One of the Starlog Special Editions on Special Effects contained an article with a couple of photos, but that's about it.
If I remember it, the original was along the lines of 8', though there was a much smaller (12") model used in one or two shots. There were two other War Rockets (one for Ming and one for Klytus). Since they weren't to be featured as heavily, I believe they were on the order of 3' long (I forgot who told me that.....)
Funny thing, I restored the smaller one back in 1989 for Van der Veer. Never took any photos and the place closed up just a little while later....sigh. Would have helped as I am working on one myself right now that has a tight deadline...........
Gene