The two base parameters for video are resize and croppping. You must treat them in that order. First resize and then crop.
Resize : with this option you can choose the image ratio and also resize it. Aspirator detects the good ratio and apply it as default. But if you're not satisfied you can change this ratio by applying the predefine ratios (set checked the "use Pred " check box) or you can put your own values to define width en heigth. Just push the apply button to make them used. Sometimes it is better to make a little resize on the image (640 for exemple). That way the result looks better and for old configs (500 MHZ) it'll work better.
Cropping : make the cropping consist in selecting a centered rectangle in the video image to isolate the part of the image you want to encode. The operation's goal (in most of the cases) is to eliminate the horizontal black bands and sometimes the little vertical ones. To do so , each time you click on the image you define a centered red rectangle wich define himself the cropped image.