you have 2 options for backups data only or full system. with data only you get all the important user data as well as application configurations if possible and then after a failure you reinstall windows and all your applications. then restore your data and settings.
full system restores allow you to boot from a media and make your system like it was the day of the backup. no reinstalling of apps and no reregistering of software... configurations are as they were.
full system can take any where from lets say as little as 70% of your current total used space to the exact size of your current hdd. the compression capibilitys of each brand of image software vary greatly as does the compressability of each different file on your system. if you have tons of text files the drive is likely to compress more than if you had mostly mp4 files as the mp4 is already compressed. some software is able to notice duplicate files and only keep 1 copy for all of them. say user.dll lives in 3 program files directorys as well as windows. some backup software notes they are all the same file and what location they all are at then keeps 1 file.
Acronis isnt a bad app ghost works well to. there are also free software that can do the job it just depends on what you are willing to spend an what features you need.