I have never used Acronis True Image to make regular images, but have used it to make backups now and again.
I assume you need to go into the Acronis User Interface and somehow get a list of the backups it has created and you can sort them by date and delete the oldest ones.
I have no idea why the backups are all such different sizes, unless it only backs up changed files, and sometimes more files are changed than others.
To be honest you are probably better off buying a large external hard drive,
1Tb (1000Mb) (or more) and backing up to that.
Backing up to the same hard disk that contains Windows is rather pointless as if that disk crashes the disk that has your backups on it will also crash.
Far better to have your backups to a separate hard drive.