Getting variables from a file

Hi, some time ago i made a pretty well working batch script to backup some data i had. But then i figured it would be cool if it was possible to add a "Backup Diagnostic" txt, that would be added to the folder destination of the files. But then i sort of crashed into the limits of what you can do in batch. Since there was no practical way to accurately measure the size of a folder. So i figured it would be fun to try and write it all again in powershell. And to make it more practical to add new folders to the backup sequence, i figured it would be better if i added all the paths to one separate text file. So my question is, how do i add my variables to a separate txt and make it add a new variable if there is a new line.