I'd have to have a look at the v-belt generator to see what the limitations are, but keep in mind that most all of the design accelerators in Inventor are based off of standard libraries. Therefore if you are attempting to create a design that would require a belt that is smaller than the library contains this isnít the correct tool.
However, the belt generator (and all of the design accelerators) is just a wizard to help automate the design of a standard component. Inventor is capable of designing just about anything, so donít mistake the limits of standard component libraries for the limits of the application.
If you are just learning Inventor youíll want to start with the fundamentals and not rely on the design accelerators to get to where you wanting to go. Put simply, design accelerators are purposed for taking the tedium out of common design tasks.
Attached is screen shot showing where to find the getting started PDF for Inventor 2009. It may help with the fundamentals. As for specifics of creating a pulley, create a sketch of a half section and use the Revolve tool to create the pulley. For the belt use the Sweep tool and sweep the belt profile along the sketched path. Look these tools up in the PDF I mentioned, and you'll be off and running.
Iíll attempt to post some helper info in a bit. Hope this helps.