This is where we slice our free space into different size partitions. You should have a plan of how your Ubuntu partition is going to be and you need to provide a suitable size for it here. Below is what I plan to do with my 10 GB free space for Ubuntu:

• 100 MB for /boot partition
• 512 MB for swap partition
• 4.0 GB for /usr partition
• The rest for root partition

Type in your new partition size. Ubuntu give hints about how you can distribute your free space using percentage here. But for me, I need to put the exact size. Ubuntu didn't mention whether we can use MB here but the default size is in GB so I guess it should be fine to use MB (The answer is YES we can use MB).