ESXi is pretty much Ligh weight. During the Installation, the ESXi would create partitions automatically. The Patitions to hold bootbank (main code), altbootbank, 4 Gigs vfat, store partition for Tools and etc., This would be enough for the ESXi to run and wouldn't require anything else. A total of around 5 G of the space is used and the rest would be left out for you to create datastores to store virtual machines.
From the left out space, Ensure that you have left at least 20 to 30 GB per VM separately to run snapshots during your patch/maintenance cycles.