At 1:1 you are viewing the image at 100% in Photoshop terms. So you are looking at it and seeing every pixel.
Now as far as print size is concerned that's a bit complicated. If you assume that the display you are looking at resolves something like 96 dpi then the image size from a 12Mp camera would be around 115 x 77 cm.
The complications comes in relating this to the real world. Most printers will interpolate an image to almost any size they can print. So whilst in theory the limit would be something like 40 x 30 cm for the same file at 240dpi, its is highly probable that the image if handle properly would print a lot larger.