On paper (ha-ha!) or in theory for nose-to-paper examination the standard has been 300dpi.
The K-x shoots [12M] 4288 x 2848 pixels
so @ 300dpi that's 14.3"x 9.5".
In the earlier days of digital photography the generally recommended printing resolution was 200dpi -
so that will make it larger at 21.5" x 14.25".
Many can't tell the difference between 200dpi and 180dpi (in fact that's the Canon default image resolution) -
so that makes it - 24" x 16".
I have printed at 160dpi and have gotten very good and detailed results (eg: 10x7.5 prints from 2Mp digicam) -
so even larger at: 27" x 18".
Of course as mentioned it depends on the details needed in the subject matter and very much on the viewing distance -
do people remember that video linked in the thread -
How can they print that big with only 12mp?
From the UK TV The Gadget Show -
where they blew up a shot from a
12Mp Nikon D700.
(I don't seem to be able to view the video now from the USA....
- but I had saved a copy back when the thread was current
Hopefully people in the UK still can see it?)
but the gist is they printed to
17x10 metres(!)
and on the video the print looked not just "acceptable",
but great even on close ups, easily beating out the ISO400 35mm film shot......