What partition size?

[armitage318]

Hi,
I have some doubt about partition size of virtual disk with Virtualization Station 3.0 app.
I am using QNAP TS-1253U-RP.
I have 3 thick volumes (17.72 TB, 9.84 TB and 7.96 TB).
I just configured one VM on 7.96 TB volume, with two disks:

- 40 GB for OS
- 7.8 GB for data

First of all, I noticed that Virtualization Station 3 lets me create even larger disk (eg. 14 TB..)
This is a bit confusing. Basically I would like to reserve 40 GB for OS and .. all free space to data partition.
How can I size this partition more accordingly?
I am afraid that I could fill space in volume before actual partition size of my virtual machine..
thank you!