Hello Bleke,
You can use the steps below to review the approximate size of your next backup:
1. In iOS or iPadOS, go to Settings > [your name].
2. Tap iCloud. At the top, you can check how much iCloud storage you've used.
3. Tap Manage Account Storage or Manage Storage.
4. Tap Backups, then tap the device you're using. Under Last Backup and Backup Size, you can check Next Backup Size. It may take a few minutes for your device to calculate Next Backup Size.
You can also find those steps in this resource: If you can't back up to iCloud - Apple Support
If that menu reports that your next backup is more than your available iCloud storage, you can use a computer to backup your iPhone instead, if you aren't interested in purchasing more iCloud storage:
How to back up your iPhone, iPad, or iPod touch with iTunes on your PC - Apple Support
How to back up your iPhone, iPad, and iPod touch with your Mac - Apple Support
Hopefully those resources put you on the path to getting your iPhone backed up.
Take care.