Optimal settings for printing

I would like to print a fews photos.

After post proces with Light Room, I would like to ask about optimal settings for transfer files from LR to memory stick and then to print.
I would like to print format 15 * 21 cm

Resize to: (And we have 5 options):

- Width/Height
- Dimensions
- Long Edge
- Short Edge
- Megapixels

Pentax K-5, II/Sigma 10 - 20 f/4-5.6, Sigma 17 - 50/2.

What are your reasons for re-sizing the images?

Carl

From Light Room I have to eksport photos so during this process I have to choice one option which is wrote above

Long edge 21 cm 300 pix resolution format 21*15

What size are your images in terms of pixels? Are they 4928 x 3264. At 300 pixels per inch to print that gives you roughly 118 pixels per centimetre- you could print 41 cm x 27 cm.
What size do you want to print is the more important question

Hi in the lightroom print module if you untick the resolution box lightroom will sort out the resolution.
Derek