Try backing up to iTunes and then seeing what the size of that backup is. That should give you an indication of how much would need to go to iCloud backup.
That's not necessary. The Next Backup Size is available in Settings > [name] > iCloud > Manage Storage > Backups > [device].
When you say that you still can't backup - do you get an error message when you try to Backup Now? If so, what is it? If not, explain your steps and results in more detail.
This sort of thing can be due to the quality of your iCloud login. Go to Settings > [name] > Name, Phone Numbers, Email (or Password & Security, or Payment & Shipping). If you're asked for your password, then sign in. If it lets you in without a password, then your sign-in was already good (and something else is going on).
Remember, automatic iCloud backup requires that you be connected to wifi, connected to charger, and the device be locked (asleep). And that you have enough iCloud storage space. Even when you click Backup Now in iCloud Backup settings, you may still need to be connected to wifi, depending on your cellular provider and/or your Cellular Data settings.