20GB is probably not enough. If 20GB is not enough, resize the Windows partition again before starting the install.
- Select the partition you want and press the
-
sign. This will delete that partition and free up space.
Create a /swap
partition. Select free space and press the +
button and set the parameters as in the image below. To find out how large this should be and to learn more about swap
read Swap Faq.
Create a /
(root) partition. Select free space and press the +
button and set the parameters as in the image below. Have at least 20GB to be on the safe side. (I have 40). For the type of partition, you can chose "primary".
You don't have space to create a separate /home
partition. So your personal files will be stored in the /
(root) partition.
Press "Install now".
However, I too recommend that you upgrade windows somehow and choose Ubuntu 20.04 LTS instead of 16.04.
After install, you will need to make sure that you create a separate NTFS partition so that you can share files between Windows and Ubuntu through it.
For more information on installing read How to use manual partitioning during installation.