I have no direct print experience with the K-5, but my 2c worth is that the maximum print size depends on the quality of the image you are printing (noise, resolution etc of the original image) as well as highly variable perceptual factors such as the distance it is being viewed from and one's personal tolerance for visible noise or softness.
Just FYI
the experts at Imaging Resource regularly do a series of print tests and give their assessment of print quality X print size for the cameras they review.
According to them...:
Quote: Pentax K-5 Print Quality
ISO 80 looks very crisp at 20x30 inches, with good color and detail.
ISO 100 through 400 also looks great at 20x30 inches, with little loss of detail.
ISO 800 starts to soften here and there at 20x30 inches, but still looks good at arm's length.
ISO 1,600 loses detail in our red leaf swatch, but other areas still look good at 20x30 inches.
ISO 3,200 looks better printed at 16x24 inches, with some luminance noise in the shadows. Overall quality looks better at 13x19 inches.
ISO 6,400 images are usable at 13x19 inches, but the luminance noise warrants a reduction to 11x14, where the whole image looks tighter and sharper. Detail in reds is quite soft now.
ISO 12,800 shots are way too noisy for 11x14 inch prints, but come right back to acceptable quality when printed at 8x10.
And so on all the way up to ISO 51200.