So I made a simple pendant, decimated it & exported it as an stl…
I typed in my measurements to 30 x 15 x 2 mm
I opened this stl file in nettfab studio to find the measurements are now 24.49 x 11.75 x 2mm 
Not that it matters in this case, but i`m wondering why & how this can be??
Edit: Seems using the slider to get one axis about right & letting it scale itself works rather than trying to change all 3 axis.