When you resize a painting to larger size, you will lose some quality. At smaller increases in size this shouldn't tend to be too noticeable, but it will become more pronounced as you start to make more considerable changes in size. The reason for this is that there is a fixed amount of starting data from the original painting size you were working with, and when you resize, this data is having to be effectively 'stretched'.
When creating paintings for physical printing, I'd recommend starting the painting at the size you wish to work with ( File -> New Painting ) as you can set the physical size and DPI so shouldn't need to resize again later.
I hope that helps, let me know if you have further questions though!
Resident Bug-Hunter and Support Guy