The maximum size does not consider "Thick" or "Thin" when calculating. The calculation is based on the current free space on the VMFS volume.
So, for example, I have a VM with 3 vDisks. For easy math let's say that the C drive is 10GB and the D drive is 20GB and the E drive is 30GB.
Let's also say that my datastore is 100GB total. 60GB is being used by my VMs (Let's say this datastore is for vmdk's only and all the other files that make up my VM are on a different datastore like my RAM and swap files etc.)
If I am in the settings for my VM and I select the vDisk that is my VM's C drive I should see that the max size for this vmdk is 50GB (the 10GB that it already is + the 40GB that are free on this datastore). For the D drive I should see a max size of 60GB (the 20GB that it already is + the 40GB that are free). And finally for the E drive I should see a max size of 70GB which of course is the 30GB that it already is + the 40GB that are free.
Hope that helps