There will be a software button for this, so that if you change one image dimension, the other tracks it automatically and accurately. The Aspect Lock is a very handy feature, don't overlook it. In Microtek ScanWizard, it is the little padlock symbol to the left of the Input dimension window. The Minolta Dual II software has the same little padlock, and it is the same thing. 11x8.5 paper is about 1.3 to 1 ratio, and metric paper is 1.414 to 1.
Aspect ratio is sometimes a fundamental consideration for sizing the image.
The area dimensions change with resolution changes. The first try isn't large enough, so you increase resolution. Do it all again. You can do it, but it's a pain. Instead just set the aspect ratio you want in the Settings Window.