Do you have the camera already? Are you loving it? I have the A620, and have been SO happy with it!
Ok, so back to your post
I think there are some different thoughts on this, but this is what has worked for me..... That dang camera has SO many settings, I'm not sure what mine are on, I'll try to remember look when I get home and update - I know its a pretty high setting so my pictures come in ~ 8.5 x 11 and 180dpi {I'm using v8 so some of the wording might be slightly different}
In your resize box, at the bottom make sure the dropdown is SmartSize and that the Lock aspect ratio box is checked. Then I change my resolution to 200 for direct printing or 300 for my LOs. Then I change my size, in this case I have 6 for my width and it makes my height 4.5.
I've used this method to have anything from a 4x6 to 12x18 printed and they have all turned out wonderfully, no pixilation or distortion. HTH