> How can Httrack be configured to only mirror remote files 
> whose filesize or data/time stamp have changed since the 
> last a mirror operation? 

This is automatically done. But the problem is generally a 
remote server problem, which is unable to fullfill the 
update request, forcing httrack to redownload unnecessary 
data.

Note: You can see the http requests/responses using the -%H 
option in httrack and see where the problem is.