As far as I know, you can't do this in Acrobat.
You can set the dimensions of the crop box in Acrobat, either manually or using a script.
This function allows you to set the crop box. When you combine this with Doc.getPageBox(), you can then adjust the crop box to e.g. the 6" area you want.
As you can see, the cConvID parameter allows you to specify the output format - and JPEG is an option.