1. It's not impossible, but probably point-less: 
It should be possible to design a seperate program to
download just enough of each file to determine its file size
and then to continue following links, but it would take
almost as long as mirroring the site itself, so it's
probably point-less.

2. Write CDs if you're low on space:  
If you're low on hard-drive space you can have Win HTTrack
pause after a certain # of Megabytes to burn a CD/DVD or
other removable media and then continue the mirroring process