It would be really helpful if Vegas told me the size of the project before I'm starting to render it. That way I have more control om customizing the project to the size I want it to be....
There is a kind of sneaky way to calculate the size of the final render. Go to tools> Burn CD> video CD. You'll see that this dialog has a file size estimator that will tell you about how big the file is going to be when finished.
This tool is there so you can adjust the bitrate to ensure that the project will fit on a CD. The estimator works pretty well with CBR compression formats, but with variable bit rate compression, it won't be very accurate.
You can use this dialog for normal rendering to hard disk- just cancel out of the CD burn process when the render is done.