I created a 3d model (suppose to be in mm) unaware that I chose the meters template, so as a result I input the dimensions way too big (e.g. 1000 instead of 1). When I imported it in other rendering software, the model is too big. I needed the model to be in mm. How can I rescale it.
Use Scale tool
Grab a corner of the model you want to scale and drag it. Type eg: 0.1, or 0.01, or 0.001, depending on what dimensions you need.
An alternative to using the Scale tool is to use the Tapemeasure tool - simply pick two points that are known to be say 10m apart [you can always draw a temporary line that long and pick its start/end] and type in 10mm + [enter], when prompted choose to rescale… and everything shrinks to suit.
Remember to change your Model Info > Units to mm and reset any Scene-tabs zooms etc…