question about my first convention coming up

hello everyone I am going to the old school expo here in st. louis next month and I have kind of a silly question to ask. I want to get tattooed while I'm there (by James Kern if I can:) ) and I was waundering if anybody knew how large of a tattoo can be completed at a convention? I have plenty of ideas ready to go just not sure what I should try to go for. Any input is welcomed. thanks to all. you guys are great