If you don’t back up anything it will take no space. The purpose of a backup is to restore the phone after disaster. If you don’t include any app data in the backup it will be lost if you ever have to restore a backup to the same phone or a replacement phone. What’s the point of backing up nothing?
Tap on Settings/[your name]/iCloud/Manage Storage/Backup/then the backup itself. This will list everything included in the backup. If there are things you don’t care about preserving in the event of disaster turn off the switches for those items. But note that this will not reduce the size of the backup immediately, because the backup is incremental and keeps earlier backed up data for a while. If you want to clear it out completely turn off backup, then delete the backup.
Note that anything you sync to iCloud will not be included in the backup; you can see what you sync to iCloud by going to Settings/[your name]/iCloud and seeing what switches you have turned on. Synced data and backup data share the same 5 GB.