There is no rule as such, of what should be the minimum size of the home partition. You can always consider how much space will/should be allocated to a user, say 2 GB, then the total size of your partition should be the number of user X 2 GB.
Keep in mind that users are restricted to their home only, so all content they create will be in the home only.
About using the existing NTFS part: it is recommended that you use a native Linux FS like ext4 or btrfs, so before using you should format the partition accordingly.
There is no rule as such, of what should be the minimum size of the home partition. You can always consider how much space will/should be allocated to a user, say 2 GB, then the total size of your partition should be the number of user X 2 GB.
Keep in mind that users are restricted to their home only, so all content they create will be in the home only.
About using the existing NTFS part: it is recommended that you use a native Linux FS like ext4 or btrfs, so before using you should format the partition accordingly.
Update your fs tab accordingly.