Hi there,
I've grown spaghetti squash twice in the past and have just let it romp away. you can eat it at practically any size. I have left them to be about a foot in length - that seems to be about right. They keep really very well - for severaly months - but they need to have thick skins first, so if you want to keep them let them ripen a bit first.
To cook, you can bring to the boil an simmer, or bake it in the oven - they can take about 45 mins. In the oven, stab them first...
sb