Lemme have a go at this.. Pretty new to printing but i am in the same EXACT position as you are now... Haha!
Just shot a family outdoor shoot using the K30 with the intention of printing big. Other than the ISO and aperture and whatever-else, from what i understand, the basics of printing are:
1) What will be the viewing distance? This is important because it will lead to the next question.
2) With the viewing distance in mind, decide on the DPI to be printed out. 100 DPI? 200 DPI? 300 DPI?
3) With the decided DPI in relation to the viewing distance, divide the picture's resolution after consideration of cropping by the DPI.
e.g. 3000 x 2000 / 300 DPI = 10" x 6.7"
And there you have it, the best largest print (subjective) if printing at 300 DPI for the native image resolution of 3000 x 2000 will be 10 inch at the longest side.
Do correct me if i am wrong... I wish to learn too so that i may start printing mine!