The system itself uses 5-10 GB with plenty of software installed. A 10-15GB /root partition should be sufficient for the system and software. How much you need above that is up to you and depends on what do you want to use Ubuntu for.
Nowadays, you can resize and move partitions with relative ease, so that partitioning need not to be final.
If I can give a one piece of advise, however, then consider creating separate partitions for /root (system and software) and /home. Although that may lead to wasting space (for example because there is space on the /root partition that you don't use), it saves a lot of hustle in the future.