Hi Richard,
From your background of using Illustrator, of course you are going to find that ArchiCAD works slightly differently. ArchiCAD is a pretty complicated program and is difficult to master just by trial and error. To save yourself heaps of time (and your sanity!) get a regular user to show you the basics, rather than trying to guess. Also work your way through the Interactive Training Guides
here, as these will answer many basic questions.
The length isn't a parameter you will find in the settings dialog box. Your problem is that you probably haven't turned on the tracker, as this displays the length of the element on plan and allows you to easily change it. Turn it on using the button circled at the top.
Select the node at one end of the wall, then simply enter the length and hit return.
Also, please take the time to add your ArchiCAD version, operating system, and basic computer specs to your signature as this will help people answer your questions quicker and more accurately.
Good luck!