I usually run an STL check on geometry before I export to zbrush, and fix anything that pops up. I've had this happen with what I'm 100% were clean meshes, so I don't think it was unwelded stuff. Scaling, I couldn't tell you. I can keep stuff more or less scaled cleanly in max, but I don't know anything about how zbrush…
I've had this problem several times. For me it's usually one of three issues: a. I've accidentally cloned one or more vertices, and its floating around inside my model (STL will NOT pick this up). b. I have a area with alot of triangles. c. Zbrush is grumpy and does it because it can ie: a characters face has some points…