Largest print size?

Hi guys. Sorry for the newbie question but I know very little about printing ...
What is the largest print size I can get from my EOS 400d camera?
Thanks!

As big as you like. Poster, billboard... the only proviso is that if you stick your nose up against a billboard-sized print then it might look a bit pixellated.

On the Cambridge In Colour web site, there's a resolution calculator. For somebody with 20/20 vision, it suggests:
350 ppi when viewing at 25cm
175 ppi when viewing at 50cm
87 ppi when viewing at 1m
18 ppi when viewing at 5m

Your 400D will give you 3888x2592 pixels. So, assuming that the picture was sharp to start with, you can make:
- a 12"x8" print at 324ppi that will look really really sharp even close up;
- a 24"x16" print at 162ppi that will look good if you hold it at arms length;
- a 36"x24" poster at 108ppi that will look good on the wall;
- a 48"x32" poster at 81ppi that will look good on the wall;
- a wall-sized print that will look good from the other end of the room;
and so on.

Try it and see. You'll be pleasantly surprised.