I think the general rule when they're small is 10-15% of their body weight. You might consider investing in a digital scale. You might get tagged at some point, but you'll probably lose your fear of it long before that happens. You can find just about anything you need to know on this forum. Enjoy your new friend!
Edit: Looks like a few people beat me to it. Also, I'd suggest describing/posting pictures of your housing so we can help you make sure you've got the right set up.