are there any requirements on the size of the ssd/hdd?
yes, you need SSD of at least 10% of size of HDD drives in your disk group. Let's say your have 1 TB HDD to add in a disk group, to support that you need at least 100 GB SSD or more.
How much space do I need for the root/ESXi?
8 GB should be sufficient enough. if it's USB flash storage, even smaller will do. because there won't be persistent scratch created by default.