The final size of image files produced using Hi-Res Production is determined by the settings that you choose in Preferences -> Hi-Res Production-> Output Settings -> Output File. The settings that effect this are:

Output Resolution:

This controls the Pixels per Inch of the produced images and should be set to the value recommended by your lab and is usually between 250 and 350 ppi.

Odd Sized Images:

If set to Output at Exact Size then all images and layouts will be produced at the exact ordered size.

If set to Fill to Next Standard Size, then the output images and layouts files will be placed within the smallest Print Paper Size that is marked as a Standard Lab Size. You should use this setting if your lab will only accept certain image sizes.  Your Print Paper Sizes are setup using the Print Paper Sizes dialog (under the Orders menu). In this case, any extra area is filled with the fill color that you have selected.

For example: If you have ordered a 4"x6" Print, but your first print paper size set as a Standard Lab Size is a 5"x7",  then with Odd Sized Images set to Output at Exact Size, the output image will be 4"x6". Otherwise it will be 5"x7".