RX use your real system as "baseline" , so: baseline size + 100-300mb,
then redirect everything
written from that baseline to a hidden space of the HDD called "Snapshot" , which size is :
Total space usage of anything written since baseline + 100-200mb
I use the word
written, because RX save everything new (tmp files, event logs, Windows Defender signatures, etc...) not only installed programs.
RX's snapshots are incremental, means if you write file called AAA on snapshot 1 , create snapshot 2 and then delete File AAA on snapshot 3; file AAA is still present on Snapshot 1 and 2.
During installation of RX , you have the choice o select which partitions to protect , more partitions you add, more space a snapshot will take.
for more infos on RX :
Rollback RX users guidelines
RX is the ultimate one for software/malwares testers , however a bad use can smash your system and wreck your partitions for good.