yes the image will be as large as the hdd, compression depoends on content... but 50% is not unheard of.

hmm, you could perhaps use cp for the linux part (perhaps pipe it through compression as above), and dd only for the bootable win part, sadly I do not know if things work without dd on the bootable WIN partition though (doubt it).
this is because WIN has some files that must be in certain points on the hdd and such. and as dd would copy everything even bad blocks... it should work great for that.
in Linux I would only use dd for the mbr (to getlilo...) then I should be able to just copy things across.
Code:
cp -avx /mnt/cd-dvd-with_linux_stuff/ /mnt/hdXYlinuxpartition/
Atleast the linux parts WILL work (IF hardware is similar enough).
I use this as backup for my linux systems.