John,
Are you still on 11?
None-the-less, checking the 18 window, it still will not work the way you want, and it should.
In 18 you can set the widths or the two end window, but it only allows one end to slide. Not also that the preview window,and a placed window, do not show correct sizing.
If you are still on 11, I'll enter the bug for 18. Let me know.
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"