I'm sure we've discussed this problem before, but a re-hash never hurts.
For any mesh that isn't a weapon (which is everything else), mesh size is determined by the DrawScale property (type float, in class Actor [Object >> Actor], declared on line 206). More accurately, any class that uses the assignment "Mesh=" uses DrawScale.
It is basically a scale multiplier: 1.0 will yield the same size mesh that is imported, 2.0 for double size, 0.5 for half-size, etc.