You don’t want to use Aimersoft or any other tool to crudely resize it, because VEAI works best if you just feed stuff in at its native resolution (sometimes, a slight downscale prior to processing can actually help). There’s a lot of information about this here and in Joel Hruska’s other posts about his Deep Space 9 project (warning: Star Trek).
This is straying fairly off-topic; I’m pretty sure there are already other topics for this.