Phil,
In Revit, each template can have as many or as few "types" of each "family" loaded, however, the default templates only have a few of the most common types loaded. As you get into Revit, you'll probably want to customize your template so that the stuff you use most is in there already.
If the size (type) you want is not already loaded, you'll have to load that type into your project from the family file.
On the file menu, select "Load from Library" (or "load family" I can't remember which), and browse through the directory structure until you find steel beams (probably under the "framing" directory. When you load the family, select the name of the family (don't double click on it in the open dialog) and there will be a list of all the available types in the bottom of the load family dialog.
Pick the ones you are likely to need to load them into your project.
Have fun,
Joe
