hi,,,, the space used by a backup is relative to the amount of gig's used on the hard drive,,,, so like mine,,, i have a 500gb hdd with around 140gb used,,,, so my full image backup is around 80-90 gig if i remember,,, this is because its compressed,,,,,,, so if you do weekly backups the program will find the old backup and just add to it whats new on the hard drive,,, so its not adding another 80-90 gig every backup if you get me,,,, this makes the backup quicker and use less space, as its only adding a week/month worth of new programs, music, pictures etc to the backup,,,,, this is suppose to be how it works,,,, but i have found in the past that it is a bit hit and miss, and the continuing backup has been corrupt for some reason and when it came time to use it it will not load back,,,,, so i delete the old one and do a fresh,,,, but this is only my personal preference as i find it works and gives me piece of mind,,,
which ever one you use there will be full instructions on there site to downland or in there forums to give guidance

hope this helps
good luck.