Oh, man, how long is a piece of string?
I can't answer for the other guys, but I find I just seem to keep working bigger and bigger until either my computer can't handle it or I lose patience with the saves and how long re-draws take. The short answer for me is big enough that I can see the detail. I was working at 300ppi for an A4 sheet, but I just couldn't add the detail I wanted, too few pixels!
Bear in mind that you can always tackle these things in pieces in separate files and then stitch them together later. I find I do this a lot with things like rotor heads and undercarriages that don't change much. I create them in separate files, save as, and then flatten them. They can then be pasted into your main pic as a single layer when the original donor file may have many layers. This helps to keep file size down a bit.
To be specific, I generally do my stuff to fit on an A3 sheet at 300ppi. File sizes tend to crawl up around the 1Gb mark once the layers start building up. It can take several minutes to open files and to do saves at this size.
I hope this helps!
Grubby.