I don't have whatever license it is that gives the STL options - but if there is a way to do it, there will probably be a tab under Tools -> Options -> Copatibility. (I seem to remember an STL tab existing here, but not sure which product configuration)
In most other file types, you have the option to select the import units. By default, however, Catia always uses mm.
If you have an STL license, the STL workbench will let you export your file in whatever units, and at whatever scale factor you want - it's in the Import and Export functions of the workbench.
If you do not have the STL license MM only. If you want to change the tessellation tolerance -- Tools - Options - General - Display - Performance - 3d Accuracy. Set to fixed and change the number associated number.