how do I use actual dimensions so when I export file as a dxf it will be correctly read by another program? I see numbers on the screen but don't know how to make sure they are read as inches or centimeters.
Which application are you using?
Studio as default uses centimetres, if you select 'create a primitive' it should explicitly list the units next to the parameters.
Primitives can be very useful for checking sizes - I've just custom-morphed a character to meet some needs, one of which was that she be about 5' 6" tall, so I created a 165cm tall cylinder primitive and scaled her against that.
I am using Hexagon, I called tech support today and the man said there is no way to control true dimensions meaning someone could not open this file in a CAD program and reliably have true dimensions
That is pretty much true for all software is it not? If I build in Googles SketchUp only SketchUp users would get the same Size. Bryce or Hexagon or any other program would need a reference of some type would they not?
Hex uses 1 unit = 1 cm, however it also has a rather small work area, the grid squares you see are 5 cm square and the whole grid area is 1 meter square, because of this what you thought was a large model might actually be tiny.
The right hand popout window has a Properties section, the Size is in centimeters, with your mesh selected you should see how big your mesh is with it.
Exporting is where your problems lie, as without knowing what the other programs 1 unit = ??? is means you'll be messing about for a while trying to get the right scale.