iCloud space needed on my iPhone

Your question is somewhat unclear. I suspect you want to achieve what I address in the first point below, but there are other possibilites.

• Do you want to expand your iPhone space by using some of your iCloud+ storage space? This cannot be done - What's the difference between device storage and iCloud storage?
• Do you want to buy more iCloud space using your iPhone? - Upgrade to iCloud+
• Do you want to access your iCloud Drive on your iPhone? - Access iCloud Drive files from your iPhone and other devices