OS Partition size recommendation?

Ideally you shouldn't need more than 150-200GB of space for the OS partition in a server environment, but as john stated 300 is a good safe round number. If you had a smaller HDD in the server I may say you'd be fine going smaller, but you're not going to miss less than 10% of the drive.

Disagree with John.  I never make an OS partition of more than 127 GB.  And I've never seen a professionally managed server need even close to that (EXCEPT for RDS servers - those are the one exception).

Now here's the thing - you said you have a server with 4 TB of space - that's for VMs RIGHT?!?!? You should be virtualizing.  In which case the hypervisor should never need close to 127 GB, let alone 300-500GB.  And your VMs should have AT LEAST 2 Virtual hard drives, one for the OS and one for the data.  And since the VMs have a separate OS virtual disk, if you ever DID need space, you just expand the virtual disk!

If you have nothing more than a Hypervisor the I understand. For standard servers with a full OS installed, you can run short with too little space.

Disk is cheap, so no need to make it too small.

I agree with John, the previous server had 1TB of total HDD and they used 150GB for OS and they ran out of space and that because a headache. thanks John.

thanks John always so helpful.

EVERY VM I create maxes out at 127 GB on the virtual hard disk for the C: drive.  NONE use more than 60 GB (even my exchange server).  Most use 25-40 GB.

You are very welcome and I was happy to help

Thank you for your kind comment

the previous server had 1TB of total HDD and they used 150GB for OS and they ran out of space and that because a headache

If you'll note, I said "professionally managed server"

Then they didn't manage it properly.

I think you can rarely assume the server will be professionally managed, though. Often times it's better to assume you're dealing with a novice so that you thoroughly cover the issue.

You shouldn't be.  If you have a server, you should have a professional or professional understanding of it.  Otherwise, you're wasting time, money, and resources.