I haven't played around with the options, but the size of the 
screencapture seems to vary depending on the avi file size etc 
... This URL has the free trial for download, and may have more 
info:
http://www.any-capture.com/
Some of my captures 
are 600 in width, others for smaller files are 200 etc lol. So at 
least that part is not set :) I seem to recall my captures from 
PowerDVD being quite large though ... must have been the 
source file/settings. I no longer have that program so I can't 
check.