atownsend advice is correct. In addition, be sure to maintain aspect ratio. To validate use the following Excel formulas (or manually)
Cell A2 width
Cell B2 height
Cell C2 is the aspect ratio formula: =A2/GCD(A2,B2)&":"&B2/GCD(A2,B2)
e.g.: 1920 1080 16:9
Copy the formula down and sort by ratio and width to keep a list of resolutions for quick reference.
For manual calculators GCD is greatest common denominator.