I have run into the same problem. Usually it scales them much larger than it should be. What I am doing is using a drawing file at 1:1 scale and "save as" DXF. I don't see a check box for view or sheet doing it that way.
1) File\Save as\
2) Change the "Save as type" to DXF
3) Click the "Options" button below the "Cancel" button
4) At the bottom of the menu that comes up there. you will see "Scale Output 1:1" Under that you will see what I'm talking about.
5) "Base Scale" If you pick the drop down you will get either Sheet Scale or View scale. Depending on the one you pick will determine what is scaled.
If you want the view(s) to be 1:1 then pick the view. If you want the Sheet then pick the Sheet. Also see the help on this.
What version are saving too? try a different AutoCAD version to see if the problem is in the R version.