This is a common question.
“How can I set exact/precise sizes in Maya?”
A simple way:
- Select the object
- Make sure you have equal scale values.
For example with Modify->Freeze Transformations (Option Box, use only scale).
- Press Ctrl+T (Universal Manipulator)
- Click on the number which you want to define (that is a distance value in cm).
It’s converted to a scale value as you can see in the Channel Box.
- In Channel Box: copy the Scale value into the other two.