Determining largest partiton dynamically

Hi,

I am using Diskpart to get information about the Windows Partition on GPT layout.

Diskpart > Sel Disk 0
                List Partition
And here diskpart shows the available partition and I OS/Windows partition is always which is largest in size ... remaining are Recovery Partition, Tools, MSR , HP_Tools (on HP System) etc etc.

Normally on GPT disks, OS partition or the largest partition is the 4th partition ... but it is sometimes 5th partition for example on Dell systems.

So need is to compare the size of all partitions and select the largest partition and change its drive letter to "W".

Can someone share knowledge if its possible in some ways?

N.A.Malik