Can anyone tell me if there is a simple way to generate a rectangular plane that is perpendicular to a line.

Extude this line on any direction? ;) (here with the options "both sides")
Then you can resize your "rectangular plane" at any size!
Or use a CPlane on this rectangular surface as background work!

Hi Wlawton, do you mean so that the line is one of the plane's surface normals?

There are a few different possible ways.

One way is to draw the plane flat on the regular viewport grid to start with, and then use the Transform > Orient command to relocate it onto the line.

See here for an example:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3660.5

And also here for more details on Orient:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3977.2

Another possible way is to use the View > CPlane command to set the construction plane to have the cplane z axis going in the direction of the line, once you have placed the cplane then that will control how the regular plane drawing commands orient their stuff. When you're done using that cplane right-click on View > CPlane to reset it back to the default world axis position.

Another way is if you drag out 2 crossing construction lines coming from the same base point on the line. When you form a sort of frame with 2 crossing contruction lines, then the rectangle from center command will align itself to that frame, see here for an example of that method:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4665.3

- Michael