Try setting a specific dimension In "picture size" select pixels as the measurement intead of inches or cm
tell it what dimension to reduce to and it will automatically adjust the other dimensions to match.
TIP: make sure to specify the larger dimension when doing this, otherwise your picture will come out dinky
If you want a simple program for viewing pics, and basic editing (crop, resize, rotate) check out ACDSee ( www.acdsystems.com ) I have the power pack, with their big editor, and I much prefer it over paint (too basic) or Photoshop (too complicated)
If anyone tries it and likes it, I can usually swing a discount as an employee