You are probably looking for something RoboHelp doesn't help you with easily.
If you hand edit the HTML code, you can insert a DIV on the page to make it adhere to a specific width.
Basically the HTML *MAY* look something like this:
This may make the editor look a bit odd as things are now inside a dotted rectangle frame. But it should work.
Thanks, Rick! I tried what you suggested and I got the rectangle frame, but not the specified width. Then I tried just adding the "<div id=body>", saved it, clicked on the dotted frame, resized it, and the code came back with the body width which I could then edit. This was SO helpful!!
Note that you can set a width directly on paragraph styles. For example:
I may not understand the problem or solution exactly, but I think this would allow you to set a width without adding an extra div to control width.