How "big" should I make my file?

[Chic]

Currently working with photoshop to make a background, will add it into illustrator when i'm done with it and finish the whole project there.
Now, the background will cover a 33x17cm box in reality, so how big should I make the background when I start the document up in photoshop?
I hope someone can help me with this, sorry if I am confusing, but I hope I managed to ask what I needed to ask
And I am also in a bit of a hurry

[eos]

you should make it that size.... 33x17cm on screen is 33x17 on print.... just make sure your photoshop doc is 300 dpi