You have Capture NX2 and can do it under Edit, Size/Resolution. Just change the Width & Height and it will change it for you. Change the width and the height will change automatically, so if you want to make 20X30 it will change the pixels to around 9000 X 6024 from a picture from my camera. Just hit the apply button. I showed this from a RAW NEF file, your JPEG won’t have such a large file size.
In the picture I show you what the picture size is and what you change it to for a 20 X 30 print. You will use 20 in the width field and 16 will audmatically go in the hight field.
I usually print 20X30 and go to Costco for a little under $10, but I want it to look it best for something I want to stand out I have it printed at National Photo labs for around $17-18 on sale. I bought a D800 last year so I am going to see how the bigger prints come out when I get back from China in spring of 2014.
A 16X20 print from Costco is $6 and $13.50 from National but about half that price on sale.
Hmmm...not really that expensive. I asked because I haven't had one done here in the US since I came to live here. Most of my old shots before my new camera were done in our own lab when I was still working so I have no idea.