You cannot resize the partitions made during the ESXi installation later. You MIGHT be able to add another partition to the system, but there's no guarantee there. I've never done it simply because I've never need to and I've always properly planned the partition sizes during installation. Either leave the default values, use the max sizes in the worst case situations where you think you'll need more space...
What have you done that has brought it up to 100% full? Are you talking about the root or swap partitions?
VCP4