Back up space
How much space do I require on my laptop to back up my iPhone 13 256gb
iPad 2, iOS 9
You could check în settings-tap your name-icloud-manage storage-backups-choose device and wait until next backup îs calculated.
This gives u an idea how big it Will be your next backup icloud. Is is a little bit different from a pc/mac backup but gives u at least an idea.
https://support.apple.com/en-us/HT204136