how to size picture to printing size?

Say I want to edit a picture to be printed out as a 8x10. How do I resize it so it can perfectly print out that size.

Don't resize it at all. Just crop it to the 8x10 ratio. Your photo is most likely in the 3:2 ratio - 3 divided by 2 = 1.5. 10/8 = 1.25
Faststone Image viewer is great for these type of quick edits. Just open a photo, hit 'X' to open the crop window and select a preset size from the pulldown menu.
http://www.faststone.o​rg/

If your using Photoshop you can also make individual Tool Presets for the various crop factors you want. It's easy to do, I've done this and it works very well.

NGC1977 wrote in post #3206556
Don't resize it at all. Just crop it to the 8x10 ratio. Your photo is most likely in the 3:2 ratio - 3 divided by 2 = 1.5. 10/8 = 1.25
Faststone Image viewer is great for these type of quick edits. Just open a photo, hit 'X' to open the crop window and select a preset size from the pulldown menu.
http://www.faststone.o​rg/

... just don't forget to set you crop "dpi" at a reasonably high number.

If you were to crop your pic at 10 x 8, but only set your dpi at 72, you would end up with an image that was only 720 pixels high, which in reality would only make a reasonable print size of about 2 1/2 inches! (divide the number of pixels by (say) 300)

TJ

TeeJay wrote in post #3209013
... just don't forget to set you crop "dpi" at a reasonably high number.

Or don't set it to anything at all. Then the software will just crop the picture as opposed to crop+resize.